While I agree that developers (like myself) are not necessarily experts at packaging stuff, to conclude that it's fine that a developer provides a flatpak is promoting shitty software.
Whether a software should run in a jail, or within user space is a decision that - for most use cases - should be made by the user.
There is absolutely no reason not to provide software as a tar.gz source code archive with a proper makefile & documentation of dependencies - or automake configuration if that's preferred.
From that kind of delivery, any package maintainer can easily build a distro-package.