I'm having the same problem. I've replied explaining that I have native sign in (that they previously had approved) that meets the requirements three different times and they have rejected 3 different times with no change to their messaging.

@Apple, please clarify more specifically. The guidelines are vague and do not explicitly state that if we have a third party sign in we also need to have apple sign in. The implication is that we can have native sign in as long as that native sign in meets requirements. If Apple is required, please state that explicitly. Many developers are being rejected unless they have apple sign in.