I don’t know if light from the rest of the sky would be intense enough to brighten those shadodws, but do remember that this is a stand alone building so the shadows on the floor are receiving very little bounce light from the building. If you had a few buildings in there then you would get both more shadows and more bounced light.
How many bounces does this method currently support, and will the number of bounces be adjustable either on a per project basis or by the end user to suit their hardware?