IIRC, cold climate specific (Mitsubishi Zubadan series for example) have COP of about ~2 at -20C. Even cheaper cold climate pumps do function at -30, albeit with COP barely above 1. So resistive heater should also be present, but it won't be used often.

In my opinion, proper winter-ready BEV should have a heat pump for heating the interior and battery, battery insulation to reduce uncontrollable loss of heat from battery, rotary heat exchanger to further save on heating ventilation air and double-pane windows to reduce losses from interior.

Double-pane windows would increase weight by about 5%, while decreasing losses through windows about 5x, iirc.