Om confused that you said the dimmer had no RFI and also that it is making radio interference below 500kHz. Have you changed something that is making it a problem now?

This dimmer works by chopping the AC signal so at less than the 100% setting you get a distorted waveform and all sorts of harmonics. The rectifier in the unit is a triac (Silicon controlled rectifier) and is necessary for the unit to function. The purpose of the variable resistor is to control the switching angle of the triac and couldn't handle the current any light would require.

A simple resistor could dim an incandescent light however they are very large and waste considerable power. What you want for dimming light is a dimmer control that has RFI filtering:

https://www.amazon.com/Legrand-Seymour-R603PWV-600-watt-Protected/dp/B0058I3SUI

When dimmers are used in stage lighting applications they typically have to carry high currents and not interfere with the sound system so the filters can sometimes be the largest part of the dimmer. A cheap motor controller may not have any consideration for conducted noise however.