The article on the Guardian about a whatsapp backdoor seems to have caused some uproar. And part of the way the Guardian did there claim was not right.
WhatsApp handles the key changes and is not a "backdoor," but is how cryptography works. Any attempt to intercept messages in transmit by the server is detectable by the sender, just like with Signal.
Today there is a article from whisper systems (from signal) and creator of the encryption system in whatsapp with a more in-depth reaction.
https://whispersystems.org/blog/there-is-no-whatsapp-backdoor/