Passkey is some sort of specific unique key to a device allowing to use a pin on a device instead of the password. But which won't work on another device.
Now I don't know if that key can be stolen or not, or if it's really more secure or not, as people have really unsecure pins.
Looks like I replied to the wrong comment.
The person you had replied to originally commented on not wanting to have the possibility of everything being broken by losing a single device. I think that's important that everyone realize that some sort of a backup plan is needed, whether that be back up codes, saving the original QR code, or being able to use multiple devices to authenticate.
At any rate, I should have replied to someone else. Sorry for any confusion.
No harm, no foul.
And you're very much right. There's a growing problem in consumer perceptions of modern security methods and how they should be applied. Much more time is spent justifying the convenience of proposed solutions, and not nearly enough time spent understanding what the scope is of the attack vectors that lead to these things needing to exist in the first place.
And further, why some of these conveniences - much like the reused-everywhere super-long password of yesteryear - are a danger in themselves.