the thing is it is supposed to be not published in steam, however it requires port fowarding in order to play multiplayer, but most people doesn't know what that is since most multiplayer games uses IP or there's a menu where you can select servers.
He put in Steam, so the players will not struggle to join in multiplayer and also, the game's networking code uses Steam to set its lobbies and players.
Also that's a dumb reason to not use Steam, it's really easy to play multiplayer with this system.
You cannot play it if you don't use Steam, why won't you do it?