Well, the example was with a tent which is a single layer of cotton between you and the environment, and by no means resembles anything even remotely insulated. That's why it needs a relatively powerful heater to stay comfortably warm. In normal use you have quite a lot of control over the temperature of the heater, mainly with the size and amount of firewood -> effective surface area of the fire.

For static buildings the situation is different, with enough insulation you can get by with almost no heating. Zero-energy building is a thing in Finland as well, and although it has its challenges, it's still possible to keep your home warm with only your body heat in e.g. -40. The main difficulty you'll encounter there is getting rid of the moisture in air, since being that energy-efficient will require having your home almost fully enclosed. You'll also need to be careful to properly limit the moisture getting out from the house, as the dew point will be inside the insulation and any moisture getting out will condense inside, eventually leading to mold.

I haven't actually reached the end game in Frostpunk 1, but at a glance it would require some efficiency improvements and better insulation – and given enough insulation and heating anything is possible. If your people are sleeping under the sky hugging the generator, I'd assume you won't get past the end game. A real-world example that somewhat resembles the Frostpunk world would be people living in Yakutsk, Russia, where they have more of a brute-force approach to the -60 °C temperatures – just burn enough gas to keep your log cabin warm. Surviving outside at those temperatures without protective equipment would be difficult though, especially since most materials won't stay flexible at -120 °C or -140 °C. You'd find it pretty difficult to move around.

Edit: But also, a mandatory "don't quote me on this", as building is my hobby, not my job. I've some knowledge, and some experience, but by no means am I a professional.