Another myth.

The fact is that the input pots must be converted from a physical position (your thumbs on the sticks) to a voltage. These days this is done by comparing voltage drop across the potentiomenter to a reference voltage. This is commonly referre to as ADC (Analog to digital )conversion. Resolution for ppm is determined by the accuracy of this ADC conversion and accuracy in the broadcast signal.

This is why PCM is also preferred where repeatability (eg. always return to the same locations) is required. Since the position is sent as a digital number (the one from the ADC), there is no pulse position interpretation of a modulated signal (PPM) required at the receiver.