I mean that they develop a editor without a specific game theme. That way you could make your own artstyle for your game. Games made for SC2 are probably going to have restrictions to suit the game client. Wc3 for example had a 4mb limit to map size. You couldn't make completely custom textures because it would take up to much space. They forced everyone to use the textures that everyone had from purchasing the game. A separate editor wouldn't have these limitations.
I dident had a 4mb limit, Battle.net had a 4mb limit. You could still play singleplayer maps made far more then 4mb. Battle.net upgraded this to 8mb with its newest patch.