The Signal Foundation does work on Signal Desktop - but they only release binaries for Mac, Windows, and Debian-based Linux distros. Those are the downloads available on their website, there is no link to the Flatpak on their website.

The community turns that official Debian release into an unofficial Flatpak release. This means that you need to trust the community packagers to be doing the right thing, along with trusting the Signal Foundation. It’s an additional layer of trust that you wouldn’t need for an official release.

An alternative option would be building the app yourself - there’s documentation here and the repo is here, but then you’re responsible for keeping up and rebuilding when they have updates. I definitely hope the Signal Foundation releases an official Flatpak, it’s not a great position to be in if you’re not on a Debian-based distro.

Just a note that the flatpak is not made by the Signal Foundation, it is maintained unofficially by the community. See the last sentence on the app description on Flathub:

This flatpak is maintained by the Flathub community, and is not necessarily endorsed or officially maintained by the upstream developers.

There’s a discussion about the community flatpak’s trustworthiness on their repo here and here, a feature request for the Signal Foundation to have an official distro-agnostic release here, but for now the only official Linux release of Signal is for Debian-based distributions.