Let's assume for the moment that the discussion of military plans on Signal was covered by this policy. That's debatable as others have said. Other parts of that policy would seem to suggest this kind of conversation is expressly forbidden on Signal and similar unofficial chat apps, while other less sensitive conversations are permitted.

How does that excuse the lack of attention and validation that resulted in an unintended party being added to the chat?

Regardless of Signal usage policy, that is a massive fuck up.