Sure Impressions only has two chords... but Giant Steps, another Coltrane tune essentially has 5 chords every three bars and modulates in that time as well. They're very different charts by the same artist. And even with Impressions they really didn't play it safe nor did they play it straight dorian in the solos either (much less so than Miles did with the same two chords). Depending on which recording of Impressions you're referring to, I wouldn't call Eric Dolphy's bass clarinet on "India" a common timbral choice either and on the same album as Impressions.

The point was over a large body of work there is much more homogeneity today than there use to be. While we can bicker about their data and study approach, the gist of their conclusion is not necessary invalid or stupid. Maybe their methods are...

[edit for clarity] Before anyone thinking about it jumps on my case... yes I know Giant Steps is 16 bars with four bar phrases or "stanzas" if you prefer... but to my ears the fourth bar in each group, while structurally important overall, functions more as embellishment and transition harmonically, melodically, and (a bit less so) rhythmically setting up the next phrase... the first three bars in each group are really the focus and point. (sorry had to get that off my chest).