You can easily do that with some extra key bindings and channel commanders/whisper groups

That is not the correct analogy. Offcourse you can customize it. Just like you can customize or mod the game.

But you won’t get the actual designs to the bicycle. You will not get the blueprints to send to a factory to create exact duplicates or with your modifications.

Releasing the source code would allow anyone to copy AND modify or extend the game as they see fit. Including all the inner logic that is normally compiled away.

Piracy or a compiled release without DRM (like GOG) only allows you to play the game and maybe modify some parts of it through modding after a significant amount of effort.