edit: oh, @ekimia sorry I partly misunderstood you post on first reading; so here a better response
The debian suggestion is one way to install Signal and I agree I personally would rather go the flatpak as way.
But coming from a vanilla ubuntu (or other OS which does not set up flatpak from the start) installing and configuring Signal and other necessary packages etc to run via flatpak is more complicated. I think going the apt way is actually more helpful to most people, because they can be assumed to know how apt works.
And your answer could have been better by actually giving the commands to install flatpak and gnome-software and making sure the configuration fits (or linking an article to do that). I just got “bitten” on my debian system that flatpak auto-updates only work with gnome-software. And does gnome-software work for KDE (or other desktop environments)? And where to put the folders? And also the Signal flatpak is not an official one, so some users might prefer to get the official packages NB: i mostly run snaps/flatpaks only without X access so those can not spy effortless on one another; but that was a moving target in the recent past - not sure if the snap worked without X or not; but for both flatpak and snap systems it needs different configuration to prevent the access (i.e. extra need for configuration as opposed to apt).
So I agree one could construct that simos does push apt rather than flatpak. But actually the apt way is the one which is better for most people out of the box. One could also construct that simos does know how to solve this in apt but not necessarily for all cases in flatpak. I think the thread only needs a good flatpak suggestion. So if you want to contribute to Signal/flatpak…