To me this is a fair rule and benefetial for the end users, it will give more options to them. It simply says "if there are competitors, you must include us".

I'm not an Apple user, but I would expect Apple to provide and guarantee that you can log in any app with your Apple login. Seems fair to me.

Also, I'm pretty confident that Google offering a privacy-oriented SSO on Google Play would be appreciated by everyone. Privacy on Android is such a joke : any app can freely read the accounts present on your phone, they don't even need you to sign in to identify you