It is really quite simple.
Flatpaks (and Snaps, and Appimages and Docker containers for that matter) are essentially designed for app developers who grew tired of distro maintainers demands to fix certain things about their build systems and their applications that broke when their apps were used on distros other than the exact distro and version the developer was using. They are designed to take a “kill the messenger” approach to the problem and now people are wondering why the work that the distro maintainers did before doesn’t get done any more.
Also to advocate for a specific tab size while also advocating for hard tabs is nonsense. The one flimsy claim to usefulness tabs have is that different people can use different tab sizes and all at the low, low cost of everyone having five times more work to use tabs for indentations and spaces for alignment and thus having to use visual whitespace of some kind.