It could be they were talking about some kind of system? LED`s in general will vary in brightness with change in current through them.
I have about 40 years experience in electronics,and an LED is basically a semiconductor,similar to a diode so the name light emitting diode. They have a forward voltage of about 1.6 to 1.7 volts,so you subtract that voltage from your source voltage,and then just use ohms law to select the dropping resistance to get the current you want. Its basically I=e/r I is the symbol used for current. So 12 volts,and 1000 ohms would give you 12 milliamps. And since auto voltage is typically about 13.8 12 volts is a good starting point with the 1.7 volts of the LED figured in.
Maybe those cars used a PWM dimmer? I know my Lincoln does.
The main thing is,even with a handful of LED`s wired in,they wont draw enough current to drop much voltage across the dimmer resistance,so either you would have to keep one or two filament bulbs in the circuit, add a load resistance in the circuit,or change the dimmer potentiometer to a higher resistance one. It will do no harm to an LED to change its brightness by vrying the current through it,unless you allow to much current to it and burn it out.