For a dev with little to no time, this does basically say you can't show other sign in options. You can have only Apple sign in, but not only $competitor sign in.

Let's say I have an old app that isn't bringing me much new revenue. I have currently have implemented two options: sign in with email + password, and "sign in with Google".

After this announcement, I would just remove "sign in with Google" as an option, and force all my users who previously used "sign in with Google" to set a password.