I agree, I suspect the following:
1. The car's heating and A/C are designed to work using DC.
2. The main heater probably uses the high voltage DC (from the main "traction" battery), to minimize currents and conversion losses.
3. The A/C probably does the same, but uses noticably less power.
4. The seat/steering wheel heater, if we have it, will use the 12-volt system, for safety. But, the 12-volt system gets all of its energy "through" the main traction battery, via a DC-to-DC converter (unless one has the PV panel, and it is adding energy to a non-full 12v battery).
5. Most likely, the only path for AC energy to enter the car will be through the current-limited charger, either 1.4 or 3.3 kW, and the charger will convert that, if needed, to high-voltage DC in an attempt to charge the traction battery, or POSSIBLY to "maintain" a given charge level.
6. The internal charger MIGHT also supply some DC energy when the battery is already "charged", and the requested pre-heat/cool begins, in an attempt to keep the battery from discharging. Of course, it could not if the EVSE was OFF, unplugged, or "timed out".
So, I suspect that the pre-heat/cool WILL use whatever energy it needs from the traction battery, and a powered charger COULD try and supply some (or much) of the energy, and that the pre-heat/cool will (MAYBE, probably) work, using the main battery, even if the car is NOT plugged in.
My 5¢ worth.
See SOC/GID-Meter and CAN-Do Info
2011 LEAF, 2014 Tesla S85
2018 & 2019 Tesla Model 3
Solar PV: 33 x 225W -> 7 kW max AC
Craigslist: Xm5000Li Electric Motorcycle