...a sorta half-assed voxel-style landscape engine?
Apparently so! The "ground" is simply a tall, narrow sprite painted to look sort of like grass at the top and then dirt and rock below it, so if it's exposed at the side of a steep gradient it looks a little like a cliff. A tree sprite is then perched on "top" of that ground. The map is generated with a terrible successive addition fractal, but it does have the benefit of being infinitely tileable, so you can fly as far as you like in any direction. All in all it actually doesn't look bad when in motion, though it falls apart if you stay in one place and rotate. I'm not really sure where I'm going with this, but there it is anyway.
Adding water, or other ground types, is easy -- it's just a different-colored sprite. Hmm, a terrain engine which only looks good when you're moving through it at high speed. I wonder what that would be good for.