The thing about passkey is the private key is meant to be un-extractable. It can be passed to keychain or google's alternative, but it can't be pulled out from either (or from the device enclave itself).

That's seen as one of the biggest benefits of passkeys for a lot of applications. But for Nostr you can't really do that, even if you wanted to create an un-extractable nsec to mimic passkeys. One reason is the iOS device enclave doesn't support generating keys with the k curve.