The article mentions the common assumption that at Coors Field the less dense air means that curve balls curve less, carrying forward the notion that Coors Field affects curve balls relatively more than fast balls.
In fact the reverse is true. Now that data are regularly being tracked on the movement of pitches and the outcomes of specific pitches, it's clear that Coors Field has a bigger effect on fastballs than on curve balls.
The reasons are not clear, but it's likely that the easiest pitch for a batter to square up is a straight fastball. It doesn't matter much how fast the fastball is; if it doesn't have movement it's going to get smacked. So the loss of movement on a fast ball has greater overall impact than does the loss of movement on a curve ball.