Comment by lorenzhs

What the article doesn't make quite clear is that OpenWhipserSystems, the creators of Signal, partnered with WhatsApp to bring their crypto to WhatsApp. See https://whispersystems.org/blog/whatsapp/ (this was when Signal was still split into TextSecure and RedPhone). The problem, of course---and the article touches upon that---is that with a closed source app like WhatsApp you have no way of knowing whether your communications are actually being encrypted; you have to take their word for it.