This was a terrible blunder. And I hope the people commenting here had the same level of outrage when Hilary Clinton, as Secretary of State, had tens of thousands of messages and conversations on SEVERAL personal unsecured, non-encrypted devices (before she wiped them) as she traveled the world. It has been established that hers were penetrated by several foreign intelligence services.
It's certainly not good what happened with this Signal arrangement, but I don't see the same level of rage and …
He was at first suspicious of the invite to join the group thinking it was probably a hoax or attempting yo entrap him. He's been interviewed extensively and in detail on many podcasts.