The linked page is on signalusers.org, but Signal's regular home site is https://signal.org/.
I'm looking all over signal.org for some link from there to signalusers.org, as that would make me more relaxed about the authenticity of the latter -- i.e., that it really is run by the same people who run signal.org.
Yes, maybe I'm being paranoid. But we're talking about an app whose whole purpose is secure communications :-).