My kneejerk reaction is the same as yours, but the fact that they were using disappearing messages - they're using Signal to get around their legal reporting requirements. Even if they have other motivations, what they're doing is illegal.

Also, I complain a lot about Teams, but my understanding is modern DoD basically runs on Microsoft, AWS, (also Google?) just the same as private companies. Probably not Zoom, which is unfortunate from a usability perspective but also wise I think.