Oh yeah, sorry just seen you mentioned me here was just going to say the topic at hand is a fairly odd one, I was interested to see what direction the video was going to go in but I don't think I have seen any other videos by that creator before, I just don't like that style of video, I'm sure the way youtube is now the core elements of his video will appear in another (20) video (s) by content creators that I find more watchable.
On topic though since I didn't mention my feelings in my first post in here, I think that the Waifu market possibly had a lot to do with it, if the characters looked less anthropomorphic then the fandom would have been more limited, I think marketing definitely had a larger part to play in the addition of breasts than the species of Squid which the characters are representing.
The answer to why they have breasts is likely the same as why they wear leather... or half a pants :o
Yeah I can see how you might find him a bit grating, and the topic is definitely odd. He's one of those people that want an in game reason for everything and won't accept "because the designers thought it looked nice" as a reason. I agree that the market definitely has an affect on these things though. Even the most lore heavy games like Skyrim take aesthetic market preferences into account. There's a reason no Elder Scrolls game takes place in Elsweyr or Black Marsh. It's because those regions are populated by furries, and not even the sort that the furry fandom would like. Sales would definitely take a hit. People want characters they can relate to, and waifus sell, therefore designers rarely stray into the unhuman or the unsexy. I've no doubt that the waifu market was the deciding factor for putting breasts on the Inklings. I normally wouldn't give a video like this a second look, but I'd seen stuff from him before and was curious what possible excuse he could come up with so against my better judgement I watched. I was very surprised by the direction it took.
The TL;DW version is that the designers deliberately promoted females in the game, either to attract more female gamers or to appeal to Western tastes. You can create a character, but there is actually a canon character design that canonically completes the story in each game, and all of them are female. The other allies in the battle against the Octolings are all female except Captain Cuttlefish. The Japanese word for the humanoid Octolings that you fight in the story mode is a portmanteau of Octopus and Amazon (like the warrior women tribes). The whole Inkling society revolves around turf wars and Splatfests for which females are the public face. He cites a 2015 Famitsu interview with Splatoon designers where the designers flat out say "the main emphasis was on the girl, to the extent that we thought at first whether it would be alright to not have a boy" among other things that suggest that the designers actually meant to promote females over males. Combine this with the fact that real life squids are massively dominated by the females (females rip males genitals off and keep them for when they feel like having kids, picking which one they want), and the fact that in game lore says that the Squidlings evolved from actual squids, and you have yourself a shockingly convincing argument that the Inkling society is female dominated. Then the lore also says that the Squidlings' ink is kept in a pouch in their chest, so there must be muscle to squeeze it out from that location. Thus the author arrives at the conclusion that female Inklings have breasts because the breasts are actually muscle, and females have more of that muscle than males because they're the dominant sex in the Inkling universe.
And the TL;DR to the whole theory is: Inklings and Octolings have breasts because it's a female dominated world and their breasts are actually muscle that lets them shoot ink from their chest pouch with more force than males.
Really the whole video might have been more interesting and less cringey if the author had called it "The Inkling World Is Ruled By Women!?", but instead he doesn't get to that stuff until over halfway through.