The cryptography that Apple uses for _their users_ is something novel and useful. I'm sure there would be completely open solutions that doesn't require Apple servers. At the moment this is Apple specific. I'm guessing most users will user Apple/Google solutions (just like how Passkeys are being used today) but for the security and privacy conscious there is always ways to keep your own private keys. Most people are handing over their private keys to Google and Apple for Passkeys today. That's not necessary a bad thing. Most people reuse passwords so it's an improvement.