"Passkeys will be importable and exportable, cross-device, and across passkey managers. They aren’t at this time, but they will be. It’s something that’s being defined and designed." -Apple Authentication Experience manger
The big providers could provide cross company SSO (e.g. sign onto my google mail account w/ my apple identity) for all their accounts already, but for some reason that hasn't happened...
Also, considering that there are already multiple software vendors providing passkey hosting functionality, it seems a little late to start designing the "cross-device, and across passkey managers" part of this. If it's not in the standard already, what assurance does a user have that any of these third-party entities will implement it?