I have no idea if that's what they're doing, but the hars edges on some of the buildings make me think so. They could also be painting texture on the 3D scene itself using something like Substance or Blender's internal painter. But getting accurate shadows with a ray traced render is a great way to build "realistic" shadows on more abstract shapes.

The railings aren't casting shadows, so I think they probably did just block out the scene with simple polygons then add those later