I’m not sure how perceptive that is. On the one hand it is a pretty common take on Letterman (including in some form by me, including relatively recently here on this list) on the other, I think it is pretty reductive and distorted. For much of his career, he reserved his real bite in making fun of guests for those he considered powerful and famous (or chasing power and fame). He mostly was less harsh with regular folks, though sometimes did cross the line, and when he did it was gross.
Roebuck’s take though is consistent with my memory of that entire movie. I really liked the book, but the film was simplistic and reductive and amounted to little more than second rate impersonations of famous people.