It’s true. Editors like to harp on the importance of quality editing. And yes, this harping is one form of ensuring job security.
David Ebershoff edited both The Orphan Master’s Son, plus Embers of War, both published in the same year, both Pulitzer winners. (Read the Q&A with Publishers Weekly here.) I don’t know his secret, but I’d suspect it has a lot to do with listening, close reading, and candor (and of course, experience, luck, and working as a novelist himself).