I think you missed Nekfarmer's point.  It's not that terraces make topsoil.  Rather, he feels they didn't have terraces to protect and keep the topsoil on site.  

I'm not sure I agree with him, but then again, I don't live in Kansas.

Reading history, you read about the dust bowl days of the 1930's. I have to wonder how much topsoil was lost from Kansas in those days from wind erosion.   Terraces would not have helped with that at all.