That sounds like a very unrealistic scenario. This is using OAuth2 behind the scenes (same as Facebook and Google login), so you only need to integrate another provider, not integrate a completely different authentication system. That means you need a couple of URLs, a secret key, and a new button on your login screen.

I'm not a mobile developer, but I do run a SaaS that has Facebook and Google login, and adding Apple login would take me a couple of hours tops. I can't see how it would be magnitudes different on a mobile app.