If the policy is "if you offer one or more authentication providers, you must include Apple sign-in", while it's still a little harsh, I think it's much more defendable and reasonable.
Again, the tech is fine. The strong-arm is indefensible.
If you support FB login now and decided to add Google, for example, that doesn't require your existing FB users to do anything different. It should only affect new users who are creating an account and choosing to use the new provider. Wouldn't that be the same for Apple Sign In?
Note, I'm not taking a position on the strong arm tactics, just pushing back on your claim regarding existing users being affected by a new identity provider. That doesn't sound right to me.