The term "AC" I think is very tricky. In looking through the technical manual, it's easy to confuse the classic AC (as in air cooling system) and the AC (air conditioning as applied to the heat pump). The heat pump warms the cabin by scavanging up all the waste heat generated by the battery, motors, electronics and circulates the AC fluid to heating coils that will warm up the air. If you are stationary, there is no heat for the heat pump to work with, and so the car has to rely on the auxiliary resistive heaters. (For cooling, I think the system uses the traditional AC system when the car is turned off. Why? I'm not sure, but the heat pump may be more geared toward removing heat from the battery/electronics/drive train, so having the "auxiliary" AC may let Audi optimize for that. Also, the car for other reasons--read below--needs a traditional AC system, so why not use it?). As best as I can tell, the "AC" on the Cabin Conditioning Display refers to Air Cooling in the traditional sense. If the ambient temp is below what you want, the car pulls in air from the outside, the traditional AC (Air Cooling) cools the air down to dehumidify it and then passes it to the heat pump system (which is using an AC in reverse) to heat the air up to the temp you desire. The preceding is what happens if 1) the Cabin Conditioning system is turned "ON" and 2) the AC button is turned ON or on ECO setting. If the ambient air is dry enough, you can turn the AC "OFF" and the heat pump portion will continue to work to heat that air up. In cold weather, you can save a few electrons by switching off the AC, but if your windows start to fog up you will need to turn the AC back ON or in ECO mode. To cool the cabin, the AC needs to be turned on, since that is the only way to get the ambient air down below cabin temp, as well as dehumidify it. Then, the heat pump adds a touch of heat to get it to where you want it. (I would think the heat pump would also have the ability to transfer the heat to somewhere besides the cabin to help it manage the battery, electronic, drive train temp, but have not delved into that at all. So, I don't know if the car has the ability to do some cabin cooling using the heat pump in reverse.)
So, in short (?), while the heat pump theoretically can both heat and cool, there are situations where resistive heaters and traditional AC systems are needed. Audi, reasonably then, just makes the best use of all three. The overall advantages are that the heat pump 1) uses 1/3 of the energy needed by resistive heaters to condition the cabin, and, 2) helps with thermal management of the battery, electronics, and drive train.
