Pretty sure that’s also accounted for, by buying multiple keys. Each person gets their own login, key, duplicate key for backup (placed somewhere offsite).

They don’t want you backing up someone elses key on your account. Again; the idea being that each person has 1 of these things. If you need a backup Yubi-Key, they want you to be buying a literal duplicate key; not having your ‘backups’ on someone elses key.