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.