But they could easily have their own F-Droid repository, I have repositories for FUTO apps like Grayjay and their keyboard, Bitwarden, and Newpipe, among others. Those are run by the projects themselves, so they’re in charge of how often they update it, as well as how they sign it. So if they have issues with the “official” F-Droid repositories, they can always host their own. I honestly prefer projects that host their own repos precisely because they should, in theory, update faster.
That said, a self-updating APK is good enough for me. However, I didn’t see an install option easily listed on their website and had to search for “signal android apk” to find the page. It should be listed on the regular install page on their website, next to the link to Google Play. I found three separate pages for getting it for Android, and all three had a link to Google Play and only one had the APK.