Both Sony/MS are more vested in the Ecosystem wars, not the old fashioned unit selling wars.

This is why you've recently been seeing Sony pushing more and more for PC users to sign up to PSN, and them eventually slapping their PSN UI onto their PC ports to "connect" PC users to PSN fully, thus making them a part of the "family" of PS. MS has been doing this for years and years, only once they got MC they started to push for it more (Bought SoT on Steam?, need an Xbox account to sign into that, same with MC).

This forum has weird outlooks on release schedules and still doesn't seem to like the idea of slimming down the port release date gap that has been getting smaller over time, because that apparently doesn't benefit the console side, despite the console side shrinking over time and generally being stagnant for some years (especially when you look at PC/Mobile growth over the past 10). 

PC folks will wait for sure, but keeping it the same schedule for a decade isn't going to net much growth and even put some people off (not everyone gives into strong-arming/Fomo, I know because I'm one of them). Shrinking the gap makes more sense, especially when you factor in the push for spreading your ecosystem, and not having one closed plastic box being your sole focus-point (Even Nintendo has been spreading out from Mobile to theme park contracts with Universal, which is a big move for Nintendo to step out of it's protective bubble after all these years). 

Developers don't have unlimited time and resources so it makes sense to prioritise the platforms where you'll sell the most.
They're already developing three versions, maybe adding PS4 would've just been that much more work for little benefit.
As has been said already, maybe the userbase left on last gen just isn't buying that many new games any more.

Do we know they are actually active users?

Is it a sign of Sony being open to offer more IPs on the Switch/2 or it’s just a sign that they wanted have a Lego brand and that was a non-negotiable part of the deal?

Leaning towards the latter.

If LEGO chose the platforms XBox and PS4 would be included. Sony signed off on this. 

You can bet your ass if Nintendo ever does a LEGO Mario game (which LEGO would say yes to in half a second), Nintendo would have full control on the game never ending up on anything but a Nintendo platform. 

Possibly, but if your consider the IPs value, for Lego, having Mario in their catalogue is definitely worth the concession they can give Nintendo in the deal they signed.

For Sony, it’s the other way around, the Horizon IP is still young and need to be a proven merch seller if they to be more restrictive in the deal they have with Lego. I’m sure the intention is to have Horizon in the Lego catalogue, but if that mean going multiple platforms on a game, that’s a fair deal.

As for PS4/XB, there can be a lot of reasons. Moving on from PS4 an XB1, maybe just a licensing issue for GP on Series X/S.

Yeah the fact Xbox is excluded suggests it was Sony's call, not LEGO's.
Considering they're now bringing their flagship games to PC, this is yet another sign that of the industry moving away from true exclusives in order to bring in more money.

I also think Sony is actually getting nervous about how older the Playstation audience is getting. The average 20 year old from the PS2 era would be in their 40s now.

They haven't been growing the younger part of their fanbase that well especially post PS2 era and I would bet their internal metrics/demographics are showing they have an aging brand. So they are trying to do stuff like this and Astro Bot as a result. 

For as much shit as Nintendo gets for making sure new generations of kids are always a priority for them, the fact is the kid who was 10 years old when the Switch launched in 2017 .... well guess what? They are 17 now. And Nintendo is just making lifer fans like nobody's business getting kids locked into their love for IP like Mario, Pokemon, Zelda, etc. early. And I see this all the time too when I'm in the Nintendo section of stores, I see majority 16-30 somethings and I know that "oh yeah Nintendo locked these customers in 8-15 years prior". 

Also today the popular culture is a lot different from the late 90s/2000s where there was an over emphasis on wanting to ditch things you enjoyed as a kid in favor of being "cool/hard/badass", today it's perfectly reasonable that to continue to enjoy things you liked as a kid into adulthood without any fuss about it. So Nintendo's philosophy in a lot of ways won out ... they played the long game and it paid massive dividends.

I think Sony is OK with Horizon on Switch because they recognize that they need to at least try and get some of that younger energy into their console platform. It's a weakness for them. They're not being forced by LEGO, they signed off on this, LEGO can't tell someone what they can and can't do with their own IP (if it was up to LEGO they would have put this game on XBox too, which obviously Sony said no to). 

If you don't get kids today when they're 8 or 9 there's a real chance you'll never have them when they're 16-25 either, they'll continue to play on Nintendo and mobile platforms. It's not 2002 anymore where there's societal/pop cultural pressure to "put away the Mario Kart and play Gran Turismo instead!" working for Sony. Today the way kids are raised it's very likely that mobile is their most played platform through childhood with a Switch as their gaming device for deeper experiences. The whole "well surely they have to graduate to a console though at some point right?" ... the Switch becomes *that* console, not Playstation. They grew up with Mario/Pokemon/etc. and Nintendo offers them a console with physical buttons and modern-style games, so that takes care of that and they continue to like Mario/Pokemon/etc. into their teens/20s/30s.

There is definitely a shift going on though it's also towards PC and not just Nintendo. Mat Piscatella recently said that zoomers and gen alpha are leaning more PC and mobile compared to prior generations and the former makes sense due to the vast majority of big gaming streamers and youtubers using that platform combined with big hits like Lethal Company being only on there while the latter is from the rise of smartphones. If this shift keeps up traditional consoles could very well become an older people thing by the 2030's or so.

Patcher always downplays Nintendo, been doing it for decades now really, obviously it's probably fair to say the Switch is incredibly popular with kids too. 

What has changed is the the whole "kid grows up with a Nintendo platform and then by age 14/15 is pushed into Playstation" ... I don't think that is happening anymore to same degree as the 90s/2000s at all and Sony knows this. Nintendo and mobile are eating up kids and changing their gaming habits young and the problem for Sony is these kids are not growing up and abandoning those platforms/IP as they age up. 

Nintendo is getting kids when they are young and then locking them in as life long fans that will be a cash cow for them as they move into their teen and adult years.

There are *a lot* of kids I think where the dynamic is --

Mobile is their 50/50 or even lead platform, Switch is actually their "console choice". The Switch is actually filling the need of scratching the itch for these gamers who when they do want something deeper/more complex with a physical controller than mobile games. 

There is definitely with this generation (and I'm seeing this first hand) a lot of kids that are fine just playing on Mobile (low level, pick me up games) and Switch (ironically this is where they go for more complex games). It's a changing dynamic.