In my opinion there are at least two ways to interpret this: a) It's an unintentional opsec failure. Perhaps there was an address book collision with another intended user. Perhaps it was fat-fingered. This seems likely. b) It was an intentional leak. Perhaps overtly, perhaps covertly, by one or more of the channel members for unknown purposes. This seems less likely as there are better ways to leak with less blowbac…
I don't see how this would work. If you're the leaker, do you just add the journalist to the group yourself? How are you going to explain that? I think there are more anonymous ways to leak stuff than adding someone else to the group chat. Or does signal not show who added someone?