Try a regular AA alkie.
The WK50 does not take Li cells, either 3V disposables or 4.2V-peak Li-ion cells. It’s strictly a boost converter, with all its inefficiencies at low-loads†.
Comparing it to a light that runs off Li cells is kind of unfair, because all that does is step down the current to the LED.
I’m not sure about eneloops, but NiCd cells would drop like a rock once they’re spent. Full-tilt one minute. completely dead the next. Haven’t used NiMH cells to characterise their end-of-charge behavior.
Alkies, though, just get higher and higher internal impedance as they’re drained, which is why those 7-/9-/15-LED cheapo lights will be “dying” when the cells still have lots of oomf left inside. They’ll drain to sub-volt levels and keep going with a proper boost-converter.
Seriously, try an alkie and see how that goes.
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† When you look at lots of switching power supplies, they’ll approach 98% or more at decent loads, but drop to 70% or less at very light loads. Most of the current drawn at that point is for the controller (and its losses), not the load.