Keep in mind, modern WiFi uses OFDM where the signal is transmitted via a number of simultaneous carriers offset by very small frequencies. So while the center of the 20Mhz band might be shifted just a little the small carries might be shifted far enough onto each other that the signal is unintelligible.
If the individual carriers aren't overlapping in the transmitter, they won't be overlapping in the receiver, either. Each one will be shifted proportionally (∆f/f0=∆v/v0) in the same direction. The signal is still intelligible. It's just shifted.
A radio is a physical device subject to manufacturing variance, temperature fluctuations, and aging effects. It has to be manufactured to tolerate frequency deviations caused by these effects. What I was trying to show in my analysis is that the effect due to Doppler shift at 10 mph is orders of magnitude less than at least one of these effects. If the Doppler shift at 10 mph makes the signal unintelligible, then the other effects I mentioned should as well.
For example, a transmitter with 50 PPM accuracy could cause a 2.4 GHz signal to drift by up to 120 KHz (0.00012 GHz). That's about 3000 times more than the 36 Hz that I calculated for Doppler shift at 10 mph.
