And what point does that happen? According to the article and other sources say similar things:

Even at temperatures approaching -30C, heat pumps outperform oil and gas heating systems, according to the research from Oxford University and the Regulatory Assistance Project thinktank.

And the lowest recorded temperature in the UK since 1961 is -27.2 °C. So the times you need to fall back to resistive or other backup systems is 0% of the time. And what do you count as a cold climate? is 0C cold? or -10C? -20C? I know many people that would say yes at any other those and I bet there are others that live in places that go way lower. Yeah, what you said is technically true, but without these numbers is almost a meaningless statement. In the UK, and most of europe this article is basically saying that heat pumps are more effective than other sources of heat even at colder temperatures and it takes extremes before they require alternative heating methods.

The way you worded your post it makes it sounds like on a average coldish winters day heat pumps become useless and there is little point in having them. Even if that is not what you intended.