Let's put it this way. Nintendo's own financial data indicates that first-party software sales are in any given quarter consistently between 70% to 90% of all software sales. That by itself should indicate how dependent Nintendo is on their own first-party output. While exclusives are still important for any console brand, they are absolutely the lifeblood of Nintendo consoles.

Also, let's look at it from a different angle. The list of best-selling games on the Switch is dominated by Nintendo titles. Here's every multi-million seller to date:

Out of 52 games on this list, only seven were neither developed by Nintendo nor were based on a Nintendo IP. The remaining 45 games on that list amount to 528.05M copies, or 46.6% of the Switch's total software sales. And that's just 45 games! There are a lot more Nintendo games on the Switch than what's on this list.

No matter how you look at it, first-party titles represent an absolutely massive share for software sales on Nintendo systems, and they utterly dominate the best-sellers list on Nintendo systems as well. This is well beyond what you see with first-party titles on PlayStation or Xbox.

SOURCES

https://www.nintendo.co.jp/ir/pdf/2019/190131_2e.pdf
https://www.nintendo.co.jp/ir/pdf/2020/200507_4e.pdf
https://www.tweaktown.com/news/91457/nintendo-made-27-billion-from-first-party-games-across-switchs-lifespan/index.html
https://en.wikipedia.org/wiki/List_of_best-selling_Nintendo_Switch_video_games

Out of the 20 Switch games that I have bought. Only 2 are 3rd party.

The percentage Nintendo lists for first party in their fiscal reports is for their revenue not unit sales in-case you weren't aware.

So for the case of the 2020 report you linked:

It says total dedicated video game platform sales are 1254.1bn yen so basically for this fiscal year the breakdown is the following:

Dedicated Video Game Platform Revenue - 1254.1 bn yen
Hardware Revenue - 654.6 bn yen
Software Revenue - 599.5 bn yen

First Party Software Revenue - 496.4 bn yen
Third Party Software Revenue - 103.1 bn yen
Digital Software Revenue - 203.8 bn yen

Obviously Nintendo gets a much better cut of the revenue of first party titles so the money spent by consumers on third party titles will be greater than the 17.2% that make up Nintendo's cut for FY20.

Using total software sales and Nintendo's disclosed first party sales in terms of unit sales first to third party is close to 50/50 on Switch.

You sure? It says "Proportion of first-party software sales to total dedicated video game platform software sales" without specifying if it was revenue or units.

In either case, even if it is revenue, it does show how dependent Nintendo is on their own first-party output. Of course, that's different than saying "People buy Nintendo systems to play Nintendo games." While that phrase is reductionist, the fact that very few third-party games do well on Nintendo systems is telling. Most Switch owners probably do have at least one or two third-party games (I have a copy of Octopath Traveler, and I downloaded Blaster Master Zero when I got the system at launch), but first-party games are clearly the main event.

Yeah, that's why they give the exchange rate underneath because they are talking about money not units. If it were units and Nintendo were getting 80% that would mean Nintendo would have sold 906m first party games on Switch from the 1130m total lol.

But yeah I think my way of viewing the situation in general is.

Nintendo: First party is the main event, Third Party is a nice extra.
PlayStation/Xbox: Third party is the main event, First Party is a nice extra.

How many times does a Switch gamer say their most anticipated game is a third party title? Or their favourite Switch game of the previous year was third party? Doesn't really happen.

There are two realities that need to be acknowledged whenever this topic is brought up.

1.  Nintendo is and essentially always has been the top software publisher in the video game industry.  This was true starting in the mid-80's, and it's still true today.  

2.  In spite of #1, Nintendo hardware doesn't sell well unless it has a good quantity of third party games.  These don't have to be AAA games, but there has to be other games there to fill in the gaps between major releases.  Usually Nintendo releases 1-3 major games every year.  If those are the only games on the system, then it feels like there are software droughts where the Nintendo system owner feels like there is nothing to play. 

Many Nintendo games have broad appeal like Mario Kart, Zelda or Pokemon.  That's why they sell a lot.  People play the other games, but they tend to appeal to more specific niches.  Some people really like Octopath Traveler and others really like Stardew Valley (for example), and there are tons of other high quality games that appeal to more niche tastes.  These games really help fill out the game library of a Nintendo system even if they aren't mega sellers like Mario Kart.

What people really want when they buy a Nintendo system is major first party releases supported by a large quantity of third party games.  This is what made the NES successful and this strategy has been present on every Nintendo system that has sold at least as well as the SNES.  Yeah, major Nintendo games sell a lot, because the games are that high quality and popular.  Third party games could sell well on a Nintendo system too if they were high quality and popular.  Monster Hunter Rise, Street Fighter II, Just Dance and most Dragon Quest games have sold really well on Nintendo systems.  However, most third party games end up filling a niche roll and are still successful relative to their budget, but they don't usually compare to major Nintendo releases.

Gotcha.

Even a roughly 50/50 split in terms of software unit sales for the Switch is absolutely massive for first-party software. Looking at the PS4's split, in FY18-19, it was only like 18% first-party, and the FY after that it was 16%. Sony's first-party games have been more consistently doing very well since at least last generation (the PS4 had six first-party games that sold over 10M copies, and two of those passed the 20M mark), but they are much smaller proportion of overall PS software sales.

Anecdotally, only including physical copies (I do have one downloaded game on my Switch, and I occasionally try out games with GP on Xbox), while only 9.1% of my Switch games are third-party, 42.3% of my PS4/PS5 games and 72.4% of my XBO/XBS games are third-party.

For me, it's not true, but with a major caveat that Nintendo is the most well-represented publisher/developer in my library, by far.

I have 80-ish boxed Switch games. Of those, 25 are Nintendo products.

I do buy a lot of third-party games, and I make it a point to buy some third-party games on Nintendo hardware to vote with my wallet for more of the same. I bought a lot of the so-called "miracle ports" like Doom, Doom Eternal, and The Witcher 3.

A lot of my favorite games on Switch are third-party games. My favorite game of 2018 was Valkyria Chronicles 4. My favorite of 2019 was Dragon Quest XI S. In 2021, it was Shin Megami Tensei V.

Nintendo is the one company whose hardware I consistently buy. I own every Nintendo system from NES to Switch. Every one of those systems has third party hardware on it, even the ones where third-party support was scarce. I bought most of Konami's N64 titles like Goemon and Castlevania. On Gamecube, my favorite RPG was/is Tales of Symphonia, a game I loved so much that I even rebought it on Switch knowing that it had performance issues (which have since been patched out). I also had Harvest Moon: A Wonderful Life, Soulcalibur 2, and Resident Evil games on Gamecube. Wii was ironically more Nintendo-centric for me than the Gamecube was despite a larger volume of software, and I only had a handful of third party Wii U games. On handhelds, third party was represented well.

However, for 20 years, between 1997 and 2017, I was a pretty PlayStation-centric gamer. Most of the third party games I liked, namely Japanese games from Square, Enix, Capcom, and Namco, migrated to PlayStation. I very seldom bought first party titles on PlayStation. That changed when the Switch came out and got some pretty solid support from Japanese third parties. I didn't really touch my PS4 much after that.

I overwhelmingly prefer Japanese games over western games as well, so there's that. Those games do show up more on Nintendo systems. There are western franchises I like. I No Man's Sky is a longtime favorite as a successor to Starlight. I like city sims. I was as impressed by Baldur's Gate 3 as anyone else. I loved Fallout 3 and NV, but Fallout 4 and Fallout 76 knocked the shine off of Bethesda for me. I enjoy The Witcher. I don't care about EA, Activision Blizzard, or Ubisoft.

Even on the NES and SNES, when Nintendo had a stranglehold on the major third parties, Nintendo games were the biggest part of my game library. Even then, I bought the NES and SNES because of Mario, Zelda, and Metroid, not because of Mega Man, Castlevania, or Final Fantasy. To me, the NES and SNES were delivery systems for Nintendo games. Third party games were the side dishes, the drinks, and the dessert that made the meal a little sweeter, but Nintendo was always the main course.

For me it's only half true. I've got a few third party games on Nintendo Switch here and there, it just depends on which run best on Nintendo systems, usually.

This is me. I would get lots of indies on Switch, but with my steam deck, I'm now getting most of it on Steam.  Mainly because the Switch has a sorry excuse for a dpad. I've even split games from the same series across the Switch and the Deck, like Blaster Master Zero, for instance.

Even with that, overall, I'm probably still 50/50 on first party vs third party on the switch.