Nitpick, but tire sales are a good proxy for determining where EVs are, since the owners are usually suckered into buying special "EV" tires.

This is addressed, though not quantified (I suppose because theres no central repository for that), in the introduction. To use your analogy, the author heard EV sales were through the roof, couldnt find any evidence that more EV's were actually on the road, so looked at tire sales to see if the answer was in there.