How they behave is of course dependent on the rest of the formula (other developing agents, pH etc), but they are not the same type of developing agent at all. Generally speaking, Phenidone is a low contrast but highly active agent at relatively low pH values, giving good speed . Hydroquinone is typically used as a high contrast agent, but typically needs to be present in higher concentrations and needs a higher pH to function as a primary developing agent. Its function in general purpose, moderate pH, fine grain film developers is usually as a "secondary" agent to regenerate Phenidone or Metol.

510 Pyro is supposed to be mixed with 0.25g Phenidone. 0.25g is a very small amount for Hydroquinone, so I don't know what it is or is not contributing in terms of image formation in this otherwise Ascorbic Acid-Pyro developer. My assumption is that without the Phenidone you'd at least have a speed loss vs the proper formula.