To be clear about the threat vector, there's also nothing stopping signal from doing the same if they wanted to. Its impossible to tell if the version of signal you download from the app store is unmodified from the code you can find on github. I trust signal more than I trust facebook, but if you use signal, even though its opensource you still have to trust them not to put anything funky in the binary they upload to apple/google.

I'd love for iOS and android to add some sort of OS-level application hash or something. "This app was compiled with xcode version X / llvm version Y with this set of options. The resulting binary hashes to ZZZ". That way with the source code you could verify that the binary on your phone is unchanged.

(Another approach would be to get apple / google to do the compilation themselves from the project on github. If apple builds my project, they could put some signed metadata in the bundle saying "We (apple) compiled this from git SHA XXX")