The comment is saying you're free to implement your own auth mechanism using plain ol email or a phone number, but you also have to support Sign in with Apple
a. actual users have a non-FB/Google OAuth identity
b. apps have something other than google or facebook login?
I've literally never encountered an app that had anything other than google or facebook for their oauth login. The only site I ever encountered that used something different was stack overflow a decade ago, and I recall them killing it off or at least complaining about it.
I suspect that the reality is that in a regular user's experience Google and Facebook login are the only ones they've ever encountered.