If you look at yesterdays livestream they show of the first Nanite demo. The demo hovers around 20,000,000 triangles in most scenes, but the actual source amount is in the billions.
I think the scene complexity you are testing on is too low for Nanite to shine. It works, but so does the old way of doing it.
PS: Virtual shadow maps works better with Nanite because they leverage the nanite structure to build shadows. Other technics need to use the nanite proxy mesh or distance fields.