From your source:

"the Mach number (the ratio of the speed of the flow to the speed of sound) must be greater than about 0.3 (since the density change is greater than 5% in that case) before significant compressibility occurs."

Key word: significant. It does not say it is not compressible, just not significant. So what everyone has been saying is correct, and there absolutely is a pressure change and a potential performance gain possible from ram air.