This leak proves that the trust in Signal is not justified. Yes, their crypto didn’t fail, but the system did. If you’re having a classified conversation electronically, you really want the system to check that the participants are supposed to be privy to this information. If some rando is in the chat, there should be a big, loud “some rando is in the chat, don’t share any secrets” alert.

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.