I would say it clearly counts as a voxel engine. Heightmaps don't support overhangs like octrees do, but the heights are in the end stacks of voxels. An octree

I would say it clearly counts as a voxel engine. Heightmaps don't support overhangs like octrees do, but the heights are in the end stacks of voxels. An octree without overhang would look exactly the same.