That's a hack. It's a login shell that, once you have already logged in and run the shell, prints a message saying you can't log in or run a shell, and then exits.
It does not stop you from, say, logging in to SSH and then starting a port forward. Or running a command in a way that bypasses the login shell. ssh will always pass it to your login shell but other ways can be vulnerable.
Do you have a script to show us how it works? Here, use mine. There's no password. Only 1 person permitted on your keyboard. If you have two people typing on it at the same time that's cheating.
It does not stop you from, say, logging in to SSH and then starting a port forward. Or running a command in a way that bypasses the login shell. ssh will always pass it to your login shell but other ways can be vulnerable.