I'm guessing olliej meant OAuth provider. FB and Google are the leading ones, so people implement those. Sign in with Apple is one as well.
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
Indeed, technically it's any OAuth provider, but in reality how many
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.
We also offer LinkedIn, GitHub, Yandex and (soon) WeChat. Most users actually use LinkedIn, but we are a recruitment firm so the stats are biased, but it's not true that only Google and FB are the options offered everywhere, at least not in this part of the world.
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