Two-thirds of households in Norway are heated by heat-pump, so I don't think that can be true.
That's a useless statistic without knowing if it's their sole heat source. Just based on costs, I'm betting there's a lot of wood stoves and pellet stoves kicking around.
That said, heat pumps absolutely do work in -20 C. I would know because i have two at home and one in the car. All three worked just fine at -25 C this past winter.
And I really do not get why there is no even option to have a diesel heaters in cold climates. It adds like 100 kilometres per long trip for a litre of fuel when it is cold and allows to use smaller and cheaper battery in EV.