We (ardour developers) have no control over what GNU/Linux distros do. You have to ask Ubuntu maintainers.
Flatpak(s) in particular are a problem for pro-audio applications. Since the DAW is sandboxed it cannot access any plugins, unless they are also in the sandbox. One solution put forward by flatpak packagers is to simply include those plugins unconditionally.
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For future reference:
binaries from http://ardour.org/download.html install everything under /opt/Ardour-<version> (plus a .desktop launch script) and come with an uninstaller.
And if you want to keep a little track of what’s being put on your system, you can run some commands like these to list what packages and snaps are currently installed, then compare.