Well this is some prize bull****. Way to select one part of the course that happens to face the same direction as the wind.
Just watched this and Phil Liggett says "they're picking up for a moment a tailwind. They've changed direction"
Here are Nairo and Froome at various points on Ventoux. Wind direction is shown by the flags, the points they are at are taken from the km sign and their position on Google Maps (finishing point is the same so km out is also the same).
Also included one of Contador + Nieve where they're riding into a headwind - worked out their position by looking at the gap to the leaders, the landscape behind them, and the fact they turn right straight after.
Shots taken 5.7km out, 5.4km out, 3.7km out, 3.9km for chasers, 2km out, 1.2km out...
Click on the photo for full-size
Therefore, this is the general direction the wind was blowing on Ventoux.. Mostly crosswind, sometimes headwind, sometimes tailwind as is always the case on a windy climb with switchbacks.
Tailwind would've been on the forested lower slopes, so not especially effective.