It isn't necessarily just a metaphor. The idea that the human population being reduced to a tiny fraction of what it once was is totally survivable isn't undisputed fact. Us surviving times where there has been a narrow amount of human life have all happened when we were hunter-gatherers or at the very beginning of things approaching civilization. The vast majority of humans today are dependent on each other and technology. The climate is also different - the sun is warmer than the last time the green house gasses on this planet rose to their highest concentrations, and we know that runaway global warming can turn a planet completely uninhabitable for human life - just look at Venus.
Yes, there are plenty of scenarios where a small amount of humanity lives on and repopulates the world over time. It certainly can happen. But there's also plenty where we don't. The author doesn't bring up any accurate science to back up his claims, or make any real arguments beyond 'We've survived population bottlenecks in the past', so I'd say that 54% of Briton's are right - climate change is a threat that could cause extinction. It isn't guaranteed to, even unchecked... but the question isn't whether or not it's a guarantee of extinction for humanity, or a threat possible of causing it. And it certainly is.