mistake in user management

in https://help.ubuntu.com/12.04/serverguide/user-management.html

the documatation cited
To disable the root account, use the following passwd syntax:

sudo passwd -l root

It is wrong when I find passwd's man page

  -l, --lock
           Lock the password of the named account. This option disables a password by changing it to a value which matches no possible encrypted value (it adds a ´!´ at the beginning of the password).

           Note that this does not disable the account. The user may still be able to login using another authentication token (e.g. an SSH key). To disable the account, administrators should use

   usermod --expiredate 1 (this set the account's expire date to Jan 2, 1970).