They are supposed to have rigorous, thorough training about what types of channels are considered secure and legal, and which types of communications are secret, top secret, confidential, etc.

Discussing war plans is top secret and if the highest ranking members of our military don’t understand that, we are all fucked. The thing is, they do know that, but they are obviously choosing to communicate on Signal so that all of their communications are off the record and undiscoverable, which is another primary reason that what they are doing is illegal and immoral.