Sadly many Linux distributions do not have user-friendly ways to install third-party applications and as a result we see instructions on running curl via sudo bash.
But again: what's the threat model here? If you're worried about someone stealing your messages, then they don't need root access -- they just need to give you a malicious build of Signal. That's way easier in an unofficial ecosystem like Snap than it is with a third-party package repository that Signal's developers are signing for.
(My understanding is that you can also configure apt to limit packages on a per-source basis, but I won't recommend that since I don't think anybody bothers to do that.)
By installing signal-desktop you give Signal not only access to your messages (which is fine), but root access to the whole system which allows reading and modifying any file. Even if Signal doesn't have malicious intents, they might have vulnerabilities in their installation or configuration scripts.