I understand what you're saying here, and I agree, BUT...
I think it's also important to understand that signal is also a company that, at some ooint, needs to make money from somewhere to do this awesome thing and they won't get that limiting themselves to an obscure app store that maybe 1 in 100 users even know about.
I also highly doubt that Google would modify the signal binary with a backdoor. When that happens, truth will come out within days and it would severely damage Google's reputation, and at least the EU would be screaming bloody murder to push new stores from independent providers immediately. It would end bmvery badly for Google, so I think there is honest doubt about that argument.
So yeah, they should at least ALSO host a version in fdroid, give users the option where to install it from
The entire thing of limited app stores is just absolutely removed in the first place, it's nice and easy (taken the idea from open source there) but implemented in a way that only AND ONLY benefits the vendor Google. Or apple, same shit, different name.
Phones should have an operating system where you can add app stores to. Linux phone, here I come!