It might be current regulated, but not to a constant level. The brightness off Preon is directly proportional to the battery voltage. This makes a Preon much more friendly for alkalines than a constant brightness light.
If the LED was on direct drive I would agree....
The Preon head can accommodate 1x or 2x AAA batteries the total voltages are below the typical Vf - so the circuit is a boost circuit.
Since the boost circuit boosts the voltage up to Vf - which remains the same for the emitter regardless of the battery used. Then the brightness of the light is dependent on the amount of energy (so current) that the batteries can deliver to the circuit/LED combination.
Does the difference between a nominal 1.5V of the alkaline (which sags much more quickly) and the nominal 1.2V of the NiMH make much of a difference? Since they both have to be boosted to Vf and therefore the brightness is dependent on the ability to deliver enough energy (so current) to drive the LED?