Signal offers an official deb. The correct behavior would have been for third parties to not distribute trademark-infringing binaries and call them Signal
Yeah, checking on IA the snapcraft page, before Sep there was no notice (one was added early Sep) this was an unofficial package and the link just below the package name was/is link to Signal homepage with tooltip saying developer. Think the issue was the lack of such notice before. Because on Flathub the equivalent package exists fine: https://flathub.org/apps/details/org.signal.Signal
Signal has historically taken issue with 3rd parties distributing binaries and calling them Signal (understandably so). Most prominently with another serial trademark violator, F-Droid
Regardless of whether you actually should* be able to, "should" and "can" don't always match.
* I'm sure Signal would object to you redistributing binaries under their name, even if you claim they are unmodified, but they can't verify that fact. And honestly such an objection seem pretty reasonable.
Modification of binaries of open source projects is a common and perhaps the only issue from a trademark standpoint. Firefox and debian used to have this too until they resolved it at some point.