And the advantage of doing this over letting application authors maintain their own packages and deptrees with flatpaks/appimage/whatever is...?

They could be based on top of Nix. In practice, it's more likely they'd be based on top of rpm-ostree. But that doesn't do anything to close the gap between the wild west of copr/ppa/aur and "get your application accepted into mainline repos" for someone who wants to distribute their app.