You're mixing up two separate things: maximum heat output (kW), of primary interest when initially heating a car (and potentially in very cold weather), vs. maintenance heating in random camping weather. The former requires either a much bulkier, heavier, more expensive heat pump than is needed for air conditioning/pack cooling, or more realistically, a resistive heat source (the latter is the setup of almost all EVs that utilize heat pumps). Normally the resistive heat source is a PTC heater, but in this case, the drive unit seems the likely answer, since a PTC is said to be missing, and Tesla already switched pack heating to be drive unit-based.
But that's only needed when you need high power output. If you only need minimal maintenance heating, there's no reason to generate resistive heating in the drive unit; you'll just use a heat exchanger with the outside air as your cold well. So bringing up the drive unit heating in this context is irrelevant.