It's not a backdoor, but it's not "not a protocol flaw" either. If The Guardian had reported it as a flaw in the protocol, would you have been okay with the coverage?
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.