Hello, author here :)

I originally chose to go with axis-aligned blocks and hard axis-aligned normals because I liked the aesthetic. I've since slightly course-corrected; each voxel has bent normals which follow the surface. How much the normals are bent is artist configurable. This has the effect of smoothing out the look of the surface when viewing from a distance, but still gives the distinct blocky look when up close.

In terms of performance, there is a cost to having fully 3D normals per voxel, but it's certainly manageable. There's a lot of other, more expensive, stuff going on.