Comment by nodamage

I think the most likely threat in this case is with ex-employees. If Bob has access to bob@example.com and creates an Apple account with it, then subsequently gets fired from example.com, they might delete his email address but his Apple account will still allow him to login to services using Sign in with Apple. (Because Apple only checks ownership of the email address when the Apple account is being created.)

Google accounts have the exact same issue so I don't understand the distinction made by the OP though.