He's using Graal, which is a bit of a mess as far as it's graphics engine is concerned. it only really allows a few/no varations on specific game objects that are placed over the normal level tiles. to get more you'd be replacing major sections of the tilemap, limiting other terrain variation.
all in all a pain in the ass, but he's doing a pretty good job with what he has. he COULD concievably use the mini bush object to add variation, but I'm not sure if he could even do that. been ages since I messed with that stuff.