Haha, just to join this philosophical discussion and to clarify things:
We (the dev team) call the mod "open world", because you can play with the same character on more than one server. Means, on 4 servers the same match i.e. the same round will be played, so actually it is one big match of 4 x 200 (if each server has 200 slots) players.
So we call it "open world", because you can travel among servers. Hope i clarified things instead of perplexing ^^