After playing a bit with it, it looks like some variant of the marching cubes algorithm, and it looks recursive, as expected. Same as your Reddit thread but in 3D.
There are most likely a bunch of optimizations, but I can't think of another way of doing it. There are raytracing/raymarching techniques too, but these look different, and they are most likely cost prohibitive for arbitrary function. There is the marching tetrahedra algorithm too, but I doesn't look like it and don't see a reason to use it now that the patent for the marching cubes algorithm has expired.
There are most likely a bunch of optimizations, but I can't think of another way of doing it. There are raytracing/raymarching techniques too, but these look different, and they are most likely cost prohibitive for arbitrary function. There is the marching tetrahedra algorithm too, but I doesn't look like it and don't see a reason to use it now that the patent for the marching cubes algorithm has expired.