What is the circuit for the PMOS driver found in synchronous buck regulator ICs?
The key is in your above-linked datasheet: 


  At moderate to heavy loads, the converter operates in
  the PWM mode with a fixed frequency of 750kHz. At light
  loads the converter enters the power save mode by
  modulating the frequency, pulse-frequency-modulation
  (PFM) which achieves high efficiency under light load
  conditions.


So it's not really operating at 750kHz and 50uA, more like one or the other. The efficiency starts to drop off below 30mA output current, dramatically so below 10mA. 



At 1mA output current, the efficiency is ~69% = Pout/Pin. Pout is 3.3mW so the device is using about 300uA from the 5V line. Still quite remarkable.