As a Guardian supporting member, I'm forwarding this directly to their editorial complaints. There are other publications that deserve my money if the Guardian refuses to stop being sensationalist.
The Guardian must retract this story, loudly and clearly.
Moreover, in the future, The Guardian and every outlet like it needs to understand that stories like this require more than one source. If you've got a legitimate story about a crypto backdoor in a popular product, security researchers will fall over themselves to comment on it.
As you can see in this case, they're falling over themselves to disavow it. In the last hour, we've added Collin Mulliner, Matt Blaze, Peter Honeyman (\o/), Chris Kanich, and Nicolas Christin --- and all we're doing is circulating the letter on Twitter. It'll pick up steam from there.
This is not a position The Guardian or any other major news outlet should ever find itself in. This situation is what the concept of "retraction" was invented to cover.
The Guardian should have inserted caveats stating that this flaw is highly unlikely to have been exploited, but I would argue that not telling the people of the flaw is as bad as telling them that it's backdoored.
The problem with yellow journalism and sensationalism is that purportedly respectable institutions spin narratives that are dangerous.
I'd be curious to see some empirical data on the impact of this story, i.e. how many users switched from WhatsApp to Signal or vanilla SMS.
It's funny, I hadn't worked out that it's pointless to cancel a subscription if I don't first tell them why.
Ironically, if they fix this, you may have kept a subscriber with them who'd otherwise have left!