An electric heater is far simpler and reliable. Since the system includes a gas engine it also is tied to the waste heat from the engine. A heat pump would have to be an "air conditioner" system with twice the life expectancy of just a straight air conditioner since it would have a 100% duty cycle.
In cold weather, electric heat can sap 40% more energy from the battery. The BEV model has a heat pump. The i3 has a heat pump. Honestly wondering if it’s possible to retrofit the BEV parts into a PEHV and reap the savings.