For normal passwords they currently seem to support third party password managers just fine, including direct integration in Safari and the iOS keyboard.
You can also export everything from iCloud Keychain to use with another password manager.
They could always use Passkeys as an opportunity to lock things down but it would be in direct contrast to what they have been doing with password management recently.
I can already hear them making the “we can’t because it will impact users security” argument in my head.