ben_w parent
While it is correct that this was a PEBKAC error rather than Signal's error, I would like to suggest that, in general, all mobile phone apps are poor choices for anything as sensitive as planning a missile strike.
I think one could design a procedure involving a mobile phone and Signal that would be reasonably secure for that kind of use case. The number one point on that procedure would be that the phone in question isn't used for anything other than secure communication.
Of course, the US government already has approved procedures and devices for secure communication, so senior official making up their own is reckless and unprofessional.