You can sideload (for now) but that forces you to enable "unknown sources" and increase the attack surface on your device, forcing you to reproduce every new release from source by hand to verify it was not compromised... or blindly sideload binaries and just hope your IP address was not served an extra special one.

Moxie -knew- this was a path almost no one has time to use safely and liked it that way. He actively blocked attempts by the community to put Signal in f-droid, so in cases where it is the system package manager, automated signed reproducible builds can be updated painlesssly. By his own admission, moxie blocked convenient and secure community package managers because he wants most installs to happen from Google Play or the App Store so he gets analytics.

In the end Molly made it to f-droid anyway and Signal has no way to stop it, but they actively discourage the use of any binaries they did not compile and cannot track and modify at any time.