Was the dish ratatouille named after someone, or did it emerge on its own? I doubt that this dish just appeared out of nowhere

The actual ratatouille is a vegetable stew, and doesn’t resemble this in a significant way. This is a “bougie” interpretation of the concept.

It’s a peasant dish as the woman chef in the movie says, made with common, available ingredients for the season. I can’t speak to the etymology specifically but it’s not likely named by or for any individual thing. It got its name by gradual consensus and diffusion.