I have over 30 years' experience in the publishing/writing industry, working as everything from an in-house editor to an investigative reporter to a novelist to a medical/technical writer to a screenwriter. I've been an independent contractor for 24 years working as a freelance editor (developmental and line editing), writer (fiction and nonfiction), ghostwriter, and copy editor.
There are a lot of editors out there with no actual editing experience working with a publishing company, newspaper, journal, or other business that requires a high level of expertise and provides training for editors. Without working in such a position, it’s very difficult to obtain and master the skills required to become a great book editor. It’s not impossible, but it is unlikely.
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.