Beyond what u/ghaff mentions, I believe the early editing work was about pacing, whether to include or kill dead-ends and expositional pieces, character arcs and biographical details that could change, choice of setting, time of day, locations, etcetera. I believe he had one change where a different character became the murderer.

Anything that would get sidenoted, he would do in second stage proofreading only. Not even worth fixing a typo if the whole page could still be axed.

The guy does a lot of scaffolding and prototyping, and some heavy refactoring.