I haven't used Signal for group chats, but given that the published screenshot [0] says "Michael Waltz added you to the group", does that not mean that the adding user (Waltz) has Goldberg in his Signals contacts? Even if it was one of Waltz's aides who handled the phone while adding Goldberg, Goldberg's contact must have existed in Waltz's phone?
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...