Well, like many things, the improvement is logarithmic. At small values, small increases make a huge improvement. I won't claim that his point is correct, but it doesn't seem unreasonable.

720p is still less than 1 megapixel, and has more than twice as many pixels as 480p. At such a small megapixel rating, the increase is certainly going to be much more profound than a 4mp to 8mp camera. But, perhaps doubling is a better metric than quadrupling.