It was referring to the ITT, not the Ventoux stage.
edit: see post #1881
The important part of that post then is "aerodynamic efficiency is NOT the same" between different individuals. It's not even a matter of determining the CdA in the aero position. It also comes down to how long the rider stays in that position on the ride. Knowing a given rider's CdA is completely useless to the power calculation if you don't know exactly how long each rider held their aero position. Seriously, trying to calculate how much more or less power a rider is producing based on velocity alone is a joke.
edit: for the cimbing w/kg you MUST know the exact rider weight, otherwise the error in that alone could be 5-10%.
In this thread you've got two people (acoggan and Alex Simmons), who combined probably have more knowledge and experience at interpreting these relationships (ie: resistance vs propulsive variables in cycling) than the entire forum combined, and yet many people still want to argue that they are wrong and you CAN determine power output with sufficient precision to then go and claim rider A produced 10% more power than rider B, or that "X" watts or w/kg is humanly impossible which therefore is "proof" of doping.
It's all b#llsh!t. All you can do is look at large sample sizes and make inferences based on trends in the numbers.