No, it's a polygon renderer of some sort. But it is quite restricted. Each "world" is a sphere, and checking again in a youtube video, it looks to me like the sphere has a regular tessellation. It's dynamic though: some of the game mechanics raise or level hills. I'd guess they found a way to special case a lot of the rasterization given the spherical restriction. Kinda like how Descent based everything off intersections of a unit cube.