Do we really think it's Bales or just coincidence?
For many, many years one of the biggest problems Fleury had was that he over-relied on his athleticism, and made no improvement in the technical aspects of his position. To name some obvious examples, we all knew his issues with rebound control, cutting off angles, and puck handling. Bales comes in, and now we can see Fleury needing to make those athletic saves far less often because he's more often in the correct position to make a good save and not give up a rebound in the first place (puck handling still needs work, though).
What's a goaltending coach responsible for? Helping a goaltender work on their technical fundamentals. Athleticism can't be taught, but technical skills can. Until the goaltending coach change, people believed Fleury wasn't willing to learn those skills. I'm not sure that the problem wasn't that Meloche wasn't teaching them to him in the first place. (See radapex's earlier post in this thread and the article it linked to for example).
If you look back at news articles on Meloche, you'll see a lot of mentions of him being a confidante, and shoulder-to-lean-on, but not a lot of mention of working on technical skills. From day one, articles about Bales have mentioned his working with the goalies on those skills, and how they can tweak what they do in order to be better. Add on top of that the fact that Bales is one of Mitch Korn's protegés, and I have a hard time believing that it's just a coincidence that Fleury just suddenly got better at the teachable parts of his profession when his teacher changed.