Something is wrong with your basic assumption that air is incompressible at speeds below 230mph. That statement is false, air is compressible at any speed. So that original statement needs to be run down and understood. I think it is causing all of your confusion.

It is vey easy to hook up a pressure gauge in an air box and drive down the road and you'll see positive pressure well before 230 mph. So someone just made a basic error when they wrote that statement.