obviously good for the bit, but the serious reason why this exists is likely because it’s a modern gtk app -> it’s meant for gnome users / made by a gnome users -> gnome is all in on freedesktop/flathub -> these users can all expect to have flatpak availability as a common method of distribution -> therefore, release it as a flatpak, so these users who already have flatpak can more easily manage their appimage-only programs
There’s a few, even just within Twitter like microblogging:
Forks of mastodon: many, but notably glitch, hometown(?)
Pleroma (and it’s popular fork, akkoma),
Misskey (and its popular fork, calckey),
there’s a ton outside micro blogging too, not just Lemmy and kbin:
https://upload.wikimedia.org/wikipedia/commons/6/68/A_view_into_the_Fediverse.png
You probably want to look into $XDG_CONFIG_HOME and the freedesktop spec documentation if you are interested in more of where these standards come from:
https://specifications.freedesktop.org/basedir-spec/latest/ar01s02.html
(and more generally)
https://www.freedesktop.org/wiki/Specifications/