Sudo Shutdown Now on Mavericks
First I would note the day of that hint - April 2001 - so a very old version of OS X

Next look at man shutdown   

-k      Kick everybody off.  The -k option does not actually halt the
        system, but leaves the system multi-user with logins disabled
        (for all but super-user).


This means that all users are kicked off but also that noone can login, which is as you saw. This means the only user still logged in is root. However in recent OS X root is not enabled.