that's an extremely reductionist way of looking at it. most of the skills of any type of storytelling transfer over to any other media. for illustration, most of the best star trek episodes were written by sci-fi novelists, not by people who only write for television, and a good part of the best game writing was done by people who are also novelists
Sure, absolutely, they transfer over. However, I'm saying they don't HAVE to be a novelist to write for games. If you don't limit yourself to novelists and people with a great appreciation for the classics (as you posited in an earlier post) then you'll have access to a larger pool of writers, many of which could be great, despite preferring to watch Toy Story rather than read Tolstoy.
writing isn't just about writing "parts" of a thing, as if you were assembling a machine in a factory. when you are writing dialogue, you need to have a good sense of character and how to portray that through characterization, how plotting works, how to integrate the theme, how to build and resolve conflict, various structures of story that work (e.g. three-act structure and similar), that kind of stuff. there's a lot that goes into good dialog besides "being good at dialog" in isolation
Those aren't skills that only novelists have, though. I'm just trying to point out that you don't necessarily have to choose a good writer purely through them being a voracious reader of novels. They may choose to immerse themselves in films, for instance.
you can acquire those skills in places beside writing novels of course (like you could also write plays, epic poems, etc.), but it's such a dominant form that almost every writer has written a novel (especially with the advent of nanowrimo), so reading their novels is often the best way to judge their skill as a writer, particularly because it's harder to get a sense of a writer based on small bits of writing (such as short stories)
I'm not sure that almost every writer has written a novel, you know. Most of the game writers I know haven't, to my knowledge. Really, one of the best way to judge a potential game writer is to play another game they've written.
what you are saying is equivalent to saying 'if you are looking for a musician for your game, and your game is a rpg, look for only musicians who have done music for other rpgs, don't look for musicians who are good in general or musicians who have never made music for a game'. you can see what's wrong with that, right? the skills of a musician transfer over, whether they are writing for movies, television, games, or anything else. same thing with writers
Perhaps I was a bit unclear and over-zealous in my initial post, however I don't think this analogy holds water at all. I'm saying by all means use all types of musician, however a musician who specialises in RPG music might well be the ticket and shouldn't be discounted because that's all they do. I'm saying be more inclusive.