A lot of tech people here are obviously unfamiliar with the history of this. They used to use Gmail for nominally unclassified communication. Several years ago they unceremoniously dropped Gmail for all purposes, without much explanation. It was mostly replaced with Signal.

I originally started using Signal almost entirely as a side effect of this transition. It was blessed as a preferred choice of the US intelligence community for unclassified comms many years ago. And a lot of classified comms if we are honest. If you worked in the US government, you needed Signal.

This isn’t a value judgement, just an acknowledgement of reality. Given this, it would be weird if they didn’t have Signal installed.

Which means it is approved by the federal government for communications.

… but not approved for communicating certain information, such as advance battle plans, no matter to whom that information is communicated; nor communicating any information permitted on that channel to people who should not be included on that channel, such as journalists — and it seems that "accidentally" adding someone who should not be on the channel would be much easier to do in a group-messaging app, as opposed to, say, the Situation Room or some other in-person, secure location.

The existence of a phone on your desk does not mean you’re approved to discuss secrets on it.

Sure, up to and including top secret?