Something that's piqued me (and my ignorance of Signal): It's not clear to me what Goldberg's Signal display name is, i.e. is it "Jeffrey Goldberg" or "JG". And who sets that? Goldberg, or the user who has him in his contacts? I mean if he were simply "JG", and it was Waltz who set that — then the question is not so much why Waltz has a prominent journalist like Goldberg in his contacts, but why does he obfuscate Goldberg as "JG"?
It's fun to imagine that maybe Waltz has in the past been a secret source of Goldberg's. Goldberg outing the chat like this would seem to undercut that theory, but it's possible that Goldberg judged the situation as so egregious that it was worth burning his source (Waltz). And technically, Waltz wasn't acting as a "source" for this story and thus Goldberg isn't obligated to give him the usual journalistic protections.
[0] https://cdn.theatlantic.com/thumbor/TcCybzsovOAQXiH4pDwlQupn...
Seems very likely that Goldberg (who confesses to being a Signal noob) set himself to “JG” so as to not inadvertently expose himself (or his source) in the event of over-the-shoulder surveillance. Still leaves open the question of how he is even in Waltz’s contact book. That said, if Waltz is a secret source, makes sense that Waltz would make a huge public show of how he would never ever be in contact with “bottom scum” like Goldberg