How? The problem he mentions at the end of the post only applies to the currently logged-in account. If you can change root's password, then you're already root, and don't need to change root's password to gain access.
Exactly how can it use the new password to become super-user? You can't assume that everyone runs only the default admin user on their OS X system.
The file includes all password hashes, including root. So crack that, then you have superuser.