They make it sound like an intentional backdoor has been introduced to WhatsApp to facilitate monitoring.
Rather, it seems like there's a weakness in the implementation, where if a message is undelivered, an attacker could trick the sender's client into sending the undelivered message to a new key they control.
That does seem like a weakness, but not an intentional backdoor as the article initially lead me to believe. I could see how someone would trade off ease of use and message delivery with security and make that call.
Yes, it could be a subtle backdoor (with limited exploitation), and yes, open source clients would be great. But real end users use WhatsApp to encrypt their private messages on a scale never before achieved, because of the usability tradeoffs they've made. I think we should bear that in mind before describing any implementation tradeoff as a 'backdoor'.