My question is basically, as soon as they are exportable across devices, then you are trusting the encryption of every actor involved, no? Such that, if there was a process to share from Google to any other company involved, then that means any company involved could be breached and the key compromised.
You could do time bound shares. But, that is ultimately going to fall back on access to Google being what secures you. Right? (And replace Google with Apple for the same general scenarios.)
Now, Yubikeys worked by never letting you get the private key off the device. As such, if you lose the device, you can no longer use those secrets to authenticate. Solution then, was to have multiple keys all configured such that you would have to lose all of them. If you needed to reestablish a brand new set of keys, you had to do that in another way. At companies, this was largely done by doing it in front of an IT person that should either know you, or validated some other identification. (A bank would similarly require you to be there in person with government ID.)
What is the article's proposed way of letting you share passkeys at will across devices and companies, that doesn't largely compromise the security?