Found something interesting. If you create a cube in Blender, and export it as USDC, then subdivision works as expected. Unfortunately, this does not work with Collada (.dae) or Wavefront (.obj). So, it must be that the USDC export in Blender produces geometry in a format that is compatible with the subdivision surface renderer in SceneKit (as does the SceneKit editor). I'm guessing this has something to do with indexed vertices.