The reason that describing dynamos as current generators is not helpful is shown by the continuation of the sentence, which draws an entirely erroneous conclusion about overvolting.
The well known facts that you can run two filament headlamps in series, in which case they will achieve normal brightness at a higher speed, or that a filament tail lamp will blow very soon after a headlamp fails are both demonstrations that a higher load impedance causes more of the emf to be developed over the load, overvolting in both cases. As has been pointed out by others, LED lamps contain electronics to condition the voltage applied to the diode to a suitable range, about 3 V. Place suitable electronics around a filament bulb and it too will be protected against overvolting even if it is not the correct resistance. LEDs are unlikely to have the impedance required by the traditional dynamo circuit, and unlike filament bulbs, which are fairly cheap replaceable components, they are also expensive sealed units, hence the electronics.
The late filament age way of protecting filament bulbs was fitting the dynamos with zener clippers, but the dynamo instructions usually include a warning that the zeners will be damaged by prolonged running without a load.
Thévenin not withstanding, the simplest, most easily understood model of a dynamo is an electromotive force (emf) proportional to frequency in series with an inductor and resistor. The potential divider formed by placing the load resistor, or bulb, across the generator terminals divides the emf between the load and generator internal impedance. Because the inductance of the inductor is also proportional to frequency the generator internal impedance increases with frequency and so the proportion of the voltage across the load reduces as frequency increases, hence the phenomenon of tending to an asymptotic current.