No, it's not weird. The columns don't have gaps because they are columns represented by a height map, which can't display arbitrary voxel geometry (unlike octrees), but that doesn't mean they can't display voxel geometry at all.

> Which is to say, they're just columns...which is (definitionally) just a height map.

Yes. A height map is representing voxel data without overhangs.

> In fact, an octree for this approach would be _meaningfully worse_

That's irrelevant. The fact remains that rendering the same height data using an octree would look exactly the same. If the latter displays voxel geometry, the former does too.