Note that Epic claimed that "Sign in with Apple" was going to get terminated, but Apple says that wasn't true:

https://www.theverge.com/2020/9/10/21431396/epic-sign-in-wit...

And IIRC it all still works anyway, so this sounds like Epic either being confused about itself or lying to make Apple look like the bad guy (more).

And bear in mind that anyone using any third-party auth (like Twitter, Facebook or Google OAuth) could have their developer accounts terminated for violating the rules, same as Apple. I guess the key takeaway here is "if you're relying on someone else's services, don't flagrantly disregard the service's rules publicly in order to start a lawsuit and then try the case in the court of public opinion", but I think that's good advice for any service.