you only see each face of the cube from one side so that's the direction of the normal, in your case you can see the back of each face too but without a normal pointing back light only reflects off of the outside.
I uninstalled SketchUp so I can't check your SketchUp file but I could probably build a working obj or fbx in Blender if you don't understand.
From a theoretical perspective, I understand. Normals and planes, and all that. What I don't see is how to add another normal to the backside of my face in sketchup. Google isn't really helping me out either. I appreciate the offer to make one, but I would like to understand how to make it work for myself.