Until Google, or Apple, or facebook, or whoever is the passkey store holder decides to put up an interoperability wall, either through neglect, cost savings, or intentional user retention.

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...

I'll believe this when it actually happens and I can audit the whole implementation stack myself. Until then, I trust Apple about as far as I can throw them. Remember when they got sued for lying in ads and their defense was "no reasonable person in Plaintiff's position could have reasonably relied on or misunderstood Apple's statements as claims of fact"? https://www.wired.com/2008/12/apple-says-cust/

"Now, FaceTime is based on a lot of open standards -- H.264 video, AAC audio, and a bunch of alphabet soup acronyms -- and we're going to take it all the way. We're going to the standards bodies starting tomorrow, and we're going to make FaceTime an open industry standard." - Steve Jobs.

It's great that they're aiming for this, but there's no reason to take them at their word until they actually ship something.

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?