Fundamentally the biggest security vulnerability in every peice of software is the end user. It does not matter how intelligently the software is designed, no amount of preparation can handle the users. That is not to say Signal has no security vulnerabilities but almost nothing can stop someone from inviting a random reporter (if they explicitly invited them). Furthermore I have a conspiracy theory of sorts, I dont think it was a mistake. I think Trumps own administration is trying to backstab him. Maybe they had ideas of becoming more powerful, maybe they thought Trump woupd reduce their power, but I feel that the amount of government leaks and just how complicated they are would suggest infighting.