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…
Obviously, Signal is not meant for this sort of thing, so it has no reason for such a feature. It’s not a failing of Signal, but it’s not fit for this purpose.