"Yes, actually there aren't so many voxels to take care of. It depends on the world size of course, but at 256x256x48, that's similar to managing a 3k x 1k 2d map in the same way."
http://www.lexaloffle.com/bbs/?tid=201
as on the same page he also says:
"Everything in the game is displayed in a virtual 128x128x64 voxel display, including the menus and player inventory"