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.

I'm much less familiar with writing fiction but there are continuity errors, abrupt jumps that just lose a reader, errors of logic, organizational problems of various kinds.

But even if you can mostly avoid that kind of thing whether in fiction or non-fiction, you absolutely need a copyeditor who will carefully look for typos, grammatical errors, spelling and capitalization of company names, etc.