On the one hand, I agree with that ...
On the other ... IMHO, one sign of a mediocre novelist is the use of too many words. Sometimes, WAY too many.
I offer L. Ron Hubbard's epic tome Battlefield Earth as an example. It would have benefited greatly from having been exposed to an editor.
And a bonfire.
After Battlefield Earth, our boy L.Ron turned his hand to the “Mission Earth” “decology” with each of the 10 volumes almost as long as Battlefield Earth. If these are an insight into L.Ron’s psyche, it was a weird, weird, bad place. An editor could only have been a positive influence.
“IMHO, one sign of a mediocre novelist is the use of too many words. Sometimes, WAY too many.”
The same thing is generally true in music too. For example, look at guitar players. Once you achieve a decent ability to play the instrument, it is not hard to improvise a guitar solo to most music, but it is hard to make a really good guitar solo. Most mediocre guitarists just try to show how technically good they are by filling up all the “space” in the solo with lots of notes, played very quickly.
That kind of works, and it can be impressive to someone watching them do it, but those aren’t the solos that anyone remembers. They don’t “stick” with you, because they have little emotional impact. So often people tell that type of guitarist to “play less”, because they are too focused on showing off to craft anything artful.