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.
The policy as it stands is contradictory with flathub practice, unless it specifies that already accepted apps are grandfathered or something like that.
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.