Servo motors: does neutral position imply a 50% PWM duty cycle?
Does 'neutral position' imply a 50% duty cycle?

No. The RC servo standard is not duty cycle despite being called PWM. It is the pulse length (actual time) not width (duty cycle) that they care about, not duty cycle.. 1.5ms is center. 1 and 2ms are the extremes.
That's what this says:

Most servos expect a pulse at least every 20ms (50Hz). For analog servos,  the pulse is not remembered and is directly used to drive the servo. The motor is only moved whenever the pulse comes in so this directly affects response and torque. Too low a frequency and analog servo will be sloppy and weak. Too high and it overheats.
Digital servos can remember the pulse and independently update the motor at a different frequency so the possibility exists that they will work with frequencies lower than 50Hz until it becomes so low the servo thinks the signal has been lost and goes into fail-safe. Some digital servos can handle as high as 333Hz. I expect yours can handle 333Hz. This was not listed.