"Der Spiegel reporters were able to find mobile phone numbers, email addresses and even some passwords belonging to [America's] top officials."
Just wildly speculating here but it feels like if you combine this with the Signal "linked devices" attack, you get a scenario where [someone] might have pwned #MikeWaltz's phone and used their access to publicly embarrass him by adding #JeffreyGoldberg to the group chat.
@cryptadamist This is an interesting bit of speculation though. I want to believe that it was the old slip of the thumb that added his number (same way I got added to a family group chat where they openly discussed my Christmas presents one year), but some other actor basically being like “see what we could do?” is definitely in the realm of possibility.