AC is understandable because you are probably talking about 5 to 10 times difference between the power input requirement of the AC vs alternator (for running the engine etc).
Your explanation of the low torque at low rpm does make sense though. My theory does not hold much water either because whether belt was slipping or not, the alternator was still putting the same amount of load to generate the necessary power. That AC can be turned on or off to feel the difference. Nobody has yet come out and said that turning on their high beam causes the car to slow down (yet!). The only loss in the power could be due to slipping belt generating excessive heat but even then that loss will not be noticeable.
Your explanation of the low torque at low rpm does make sense though. My theory does not hold much water either because whether belt was slipping or not, the alternator was still putting the same amount of load to generate the necessary power. That AC can be turned on or off to feel the difference. Nobody has yet come out and said that turning on their high beam causes the car to slow down (yet!). The only loss in the power could be due to slipping belt generating excessive heat but even then that loss will not be noticeable.