If Instagram is using https://developers.facebook.com/docs/facebook-login/ with the little “log in with Facebook” button, it’s a social login. If they have their own proprietary way to do it in-house, it’s not, and not subject to Sign In With Apple.
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.