This has been their policy but now it's more explicitly defined. Go look through the submissions on https://github.com/flathub/flathub/pulls?q=is%3Apr to understand how bad the problem they are dealing with is.
It's weird because I am sure that a large number of established apps in flathub, such as.. Chromium https://flathub.org/en/apps/org.chromium.Chromium has AI-assisted code, and this bans all AI-assisted code
The policy as it stands is contradictory with flathub practice, unless it specifies that already accepted apps are grandfathered or something like that.
After some time it's looking like AI is becoming just another tool in the developer toolbox. So we can confidently say that, over time, only projects with specific anti-AI policies will be free of AI code (and actually, even such projects I'm not sure)
So it's basically a ban on most new projects, made by inexperienced developers. Which is okay I guess, if this means they can better review projects.
The policy as it stands is contradictory with flathub practice, unless it specifies that already accepted apps are grandfathered or something like that.