They'd need to be building reproducibly[1], which is somewhat difficult to do on iOS thanks to code signing and App Store encryption[2]. Android is definitely closer[3].

[1] https://reproducible-builds.org

[2] https://github.com/WhisperSystems/Signal-iOS/issues/641

[3] https://f-droid.org/wiki/page/Deterministic,_Reproducible_Bu...