While there is some credit to that, reading comprehension is falling in the U.S. rapidly. We have to look no further than newspapers to see that. When I was little (90's), newspapers were written to an 8th grade standard. Now, newspapers are written to a 6th grade standard, and many large cities are considering, or have already gone to, a 4th grade standard.
As for writing, I can see that with new writers and part time writers, but most professional writers I know that work on technical matters (manuals, rules, etc.) are able to fix 90% of their issues on their own. It's just something you teach yourself to do as you go through rewrites. Most editors don't have enough background in an area (I work primarily on software manuals for example) to understand issues caused when they change certain words.
Fiction is, of course, the easiest thing to edit.