Maybe dimmers that had variable transistors worked with the fans back in the what? 90's? but today, they only use the variable transistor as a mechanism to make the overall dimmer work the way want it to ( to not waste energy/cause unwanted heat). So the variable transistor doesn't complete the dimmers main funtion, just aids it.
So when I said Dimmer, I meant what common people know as a light dimmer.
When I said rheostat, I meant Potentiometer/Variable Resistor/Rheostat. There is and isn't a difference, it depends if you play semantics or not.