It seems to me that he has neglected the factors of increased air density and fuel density on combustion efficiency (effect of mild supercharging increases power per stroke), and decreased road friction/stickiness, both tending to increase mileage. Before on-board computers, we used to have to retune our engines for best operation according to the weather.
A modern closed loop fuel injected engine will increase the fuel delivery in response to the increase in air density, so that variable evens out. Having to push through heavier air is a bit harder for a car to compensate for. Bottom line, cold weather kills fuel economy. I keep very good records and mine has gone from 32 in the summer to 28 in the winter, down to 26 if I am fighting heavy snow or temps under 0ºF.