Seems a bit petty to me. As a developer I empathize with the possible lack of documentation (I don’t know, I never tried to integrate Sign In with Apple) but as a user I actually am super happy with it. Nowadays any iOS app that doesn’t support logging in with Apple makes me think twice about whether I really need the app.
I'm the opposite. I NEVER use a third party login, my it be Apple, Google, Facebook or whatnot. If your website/app doesn't offer their own independent login, I don't even think once whether I really need it.
I run my own mail server so it's easy for me to implement vendor-specific email addresses that cannot be correlated with other vendors', but more and more companies are offering that as a service nowadays, DuckDuckGo most recently:
https://www.spreadprivacy.com/introducing-email-protection-b...
I do use GitHub federated login, but only for apps like vulnerability scanners that do need OAuth access to my repos.