No you're completely wrong. The server loads all the map data from the map#.dat files, sends that data to the client, and the client uses the data to draw to the screen. The maps are saved on the client side for faster loading (I think), so that it doesn't have to get all the data packets from the server, unless the map was changed (Uses a revision number to check I think). The server does use some things that would be useful for an offline map editor, such as the saving and loading code.