I would imagine the doujinshi is allowed as long as it’s still fan made. For instance the Wikipedia page has a fan made cosplay of Squirtle. By the same logic as long as the manga’s depiction of Squirtle is also fan made (or drawn) then it also seems to get a pass.
It would seem the issue is less how their assets are used and more that their specific assets are used.
That’s more of an issue with their algorithm. The thing I think most people here (myself included) miss is that they used to let you know the rating on videos before you clicked them so you could see whether a given video was decent or not and therefore whether or not you should bother with some clickbait BS or not