The plugin registers in the File menu. The entry is greyed out unless there is an open image.
This is a just-installed Mint 20 VM Gimp flatpack.
Ofnuts plugins when un-zipped should already be set as executable, but check anyway.
I have installed flatpak Gimp before, never noticed that it ran as superuser, I must check that. Gimp just opened up from the Mint graphics menu.
Not much more I can really add. You can add a more convenient plugins location than ~/.var/app/org.gimp.GIMP/config/GIMP/2.10/plug-ins but not essential.
The flatpak does (should) come with python support. When you look in the Filters menu is there a Python-Fu entry. That shows there is python support.
Which linux distro are you using ?
Edit: It looks like the 'as superuser' is a issue with the MATE DE - it is a known issue where MATE's window manager Marco is misreporting that Flatpak applications are running as root when they aren't.