I think you are right.  Seems I had this discussion with Gordon once, and learned that they use the full correct mathematical expression for airspeed including all aerodynamic and compressibility effects. 

 

Being old fashion, I still have an E6B, and I know it is wrong as speeds increase because of GIGO: Garage In Garbage Out, the garbage being the OAT read in the airplane which is above the true OAT.  I went through the exercise some years ago, and attach some charts with some results in terms of correction factor vs. E6B calculated airspeed.

 

Fred