The quoted image does not say so

It does exactly say so. Flatpak is the only supported and official method of installation when you’re not using Ubuntu.

As for the Fedora issue, that is a completely different thing. That is also Flatpak, so its not the package format itself the issue.

Exactly. And the Flatpak version from Fedora was unusable.

So where does the developers say that anything that is not their official Flatpak package is “borderline unusable”?

They don’t. It’s just unsupported.