You are of course correct Old Smokey, aerodynamic forces are related to EAS rather than to CAS.
I deliberately left out the difference between EAS and CAS to keep the explanation reasonably simple. Let's face it, this difference is a very small factor compared to the difference between CAS (or EAS) and TAS at high altitudes.
The heart of the matter is that just getting a high TAS is not enough to get best range. We also need best engine fuel efficiency.