This is the first time I ever find myself kind of disagreeing with the Mint team. As others have said, some of the most popular packages on Flathub are unverified so popular programs like Inkscape are not going to show up as Flatpaks?
I think just a warning, like what Flathub does, and maybe a dialog before installing, warning the app is packaged by an unverified packager, would have been enough.
If a new user installs malware from flathub while trying out mint for the first time, they’ll probably blame mint instead of flathub. Nobody will say “damn, I should have listened to that warning” while their “discrod” app rm -rf’s their entire PC away, they’ll instead claim Linux is crap and go somewhere else. Doing this helps keep mint safe, and definitely encourages unverified FOSS apps to hurry up and get verified.
That sounds suspiciously similar to the kind of gatekeeping Apple is doing.