There is no WhatsApp 'backdoor'
It's remarkably hard to tell if a key has changed with a peer in signal.  It gives you some small faded text between messages which is easily scrolled off by the other party making a lot of comments.Once its off your screen, there is no way to tell that you've never authenticated the current key.  No mark, not even burred in a menu.So perhaps I should not surprised to see the authors of the worst public key management security that I've ever actually used defending even worse public key management security.