My understanding here is that the main objective is to break up the MasterCard/Visa duopoly. I guess breaking up duopolies is always a win for the general public, no?
There has been a rust-library for signal in the works. I read some discussion on it that made it clear the aim was to develop a proper signal-desktop application. The rust-library has a cli, which is not quite finished.
https://github.com/whisperfish/presage