I'm not interested in the political angle of this story, but I nerd out on the cybersecurity.

The act of sending military strategy to a journalist who responsibly disclosed the leak is not that big a deal. It's a UX problem for Signal. I have made the same mistake. In fact, on that same day, I thougt I sent a private message in a group chat, but I sent it to my wife. It wasn't a big deal, She's in the group chat I meant to send the message to anyway, but it means Signal is not suitable for tin-foil hat level security.

I would never send two multi-signature bitcoin keys with Signal because of this UX, zero day exploits, etc.