Signal only provides a script for .deb based distros on their official website. The flatpak is currently not ideal because it stores encryption keys in plaintext.
The provided link suggests an automated installation in a Ubuntu Distrobox including automated updates. Useful for every distro that does not natively support .deb packages.
Yeah unfortunately i can’t quite recall the context, but I think they were attempting to make encrypted storage the default, but then that broke on existing databases or something? It was a pain at least, I know that much 😅
(Although would be less of a pain nowadays, now that Signal has proper sync to restore my history.)