Your threat model is being authoritarian + a moron.
They're using Signal with disappearing messages instead of official channels because they don't want their conversations documented for accountability. It's the same shit they relentlessly blasted Hillary for.
I was most disappointed when I read that he left the group chat. Missed opportunity for some top-class trolling:
We are good on OPSEC
Are you really tho? 👀 🥸 😱