The 500V is across the cap, not the dropper resistor feeding it.
The steady state Vdc across the resistor will depend on the current being drawn by the circuits it is feeding, eg phase splitter and preamp.
The instantaneous voltage across the resistor, eg at start up, may be a lot higher than the steady state voltage, hence the benefit of the high voltage rating. Itís only going to be needed momentarily, until the cap charges up.
Specifically, in 0.5 sec, the cap will charge up to ~350V, and will then follow the curve to its eventual steady state voltage, see http://referencedesigner.com/rfcal/cal_05.php