[gwright]

I'm speculating here, but Apple Sign-In for Android would work just fine if the sign-in process was based on an OAuth flow where the credentials are entered into a web form. From the limited details I've seen that sounds like how Apple Sign In will work.

A service supporting alternate identity providers via OAuth (Facebook, Twitter, Google, Github) via a flow like this shouldn't have trouble with Apple Sign In from a web page, iOS app, or Android app.