I don't see how this makes any sense. An AC compressor is an extra load regardless of whether the motor is powered by gas or electricity. If it requires 10 Joules to run the AC for 1 minute, you need 20 more Joules worth of fuel. If turning on the AC didn't decrease gas mileage, then the efficiency of the engine somehow magically increased when it starts turning the compressor.

This same reasoning should also apply to charging your phone, cranking the bass, etc, because that represents an additional load on the alternator which must be powered by the engine.

Of course, turning on the heater in gas vs electric car is a different story, because the gas car is using the heat energy you mentioned that would otherwise be wasted.