Maybe I don't understand what you mean by the word "feasible" – they don't have a goal of getting their living room above 23 C at most in winter, and I guess heat pumps are insufficient in such a house if you desire ambient temperatures above that. However, while other means of heating could plausibly bring the temperatures higher, that would end up being very expensive also because of the poor insulation – it's just harder in general to heat a drafty house and keep the temperature up, and I don't see how heat pumps are a uniquely bad choice for homes like that.
Edit: This is coastal Norway, so the climate in winter is quite similar to somewhere like Edinburgh, with temperatures usually above 0 C in January. The heat pumps would probably be insufficient somewhere the temperatures regularly reach -10 or -20 C, but that's a very infrequent event both here and in the UK.