It's not that easy. Instagram belongs to facebook. So logging in with facebook is not strictly a "social login". Same with Tinder, IIRC they only had facebook login, to piggyback on the fb profile. That's not really social as in "chose one of those providers".

If it "chose provider" then yes, apple could shove in their own with the pretense of user privacy, while it is painfully obvious that it's just leveraging their appstore control to grab market.

Not even OAuth can be unambiguous. I can have a single OAuth login. That could be myself as provider, or leveraging some other provider, which could be more or less connected to my app.