It was not about warming quickly, it was because Beetles have an air cooled engine. The engine's waste heat is thus difficultly recyclable compared to a liquid

It was not about warming quickly, it was because Beetles have an air cooled engine. The engine's waste heat is thus difficultly recyclable compared to a liquid cooled engine which allows circulating the coolant through a heat exchanger to heat up the cabin's air. Also the Beetle's engine is rear-mounted so even if you could heat up the air with fins on the engine block, bringing heat to the front of the cabin would require much piping making it impractical and ineffective. Hence the gasoline heater. Similarly, electric cars don't produce waste heat and so the heater also gets its power from the same power source as the engine, the battery.