Here is where that plugin came from: https://forums.alliedmods.net/showthread.php?p=1850358
There are only a select few commands that the server can force the client to execute, and quit is not one of them. In fact, that plugin doesn't actually support running client-side commands, it only supports "fake" client commands. Meaning, it makes the server think that the client ran a command. So it's only going to work on commands that run server-side, like "kill".