It is a program that reads unstrusted binary blobs (many document formats) using C++ deserialization code that has a history tracing back to the nineties and even eighties (through StarOffice). I sure as hell want such an application to be sandboxed and ask for elevated permissions. This is pretty normal on other platforms like macOS (Office and iWork from the Mac App Store run sandboxed), iOS, and Android.

Furthermore nothing is fundamentally stopping anyone to apply SELinux policies to this application just like a flatpak would.

You need more than policies. E.g. if a program cannot read outside its sandbox, you need some way to mediate access to files/directories on the user's request, which are provided by e.g. Flatpak/XDG desktop portals.