"fdroid repos" have little to do with what people consider F-Droid, they can host any whatsoever binary.

In fact, that's not a build by the Guardian Project, but (when I tried) a redistribution of Signal's https://github.com/signalapp/Signal-Android/releases builds.

I'm not sure why they're doing it; anyhow, I'd at least avoid doing the initial installation through that repo, you're trusting an additional party for no gain that I could think of (updates are ok because the signature needs to match the one of the installed version).