You have not read the guideline carefully or have misinterpreted it; nothing in it locks the end-user into Apple's ecosystem. The requirement is only that if the app uses a third-party authentication system (Facebook, Google, etc.), it must also offer Sign In with Apple alongside it.

You're free to roll your own sign-in system, in which case you are not required to offer Sign In with Apple.

Not seeing a problem here.

FOSS person too. Don’t see the lock-in problem here other than indeed using Apple products. While you do, making sure you don't have to give identifying info or bother with generating disposable addresses yourself, is actually a good thing IMHO.