I am not exactly a professional in this topic (Well, yes I should be, but I lack further experience 
), but I do not think that a FET must always work as an on-off switch.
Even the highest mode could be PWM controlled. Usually direct drive(ON-OFF) is called Turbo mode in these flashlights.
I tested it with 35E Samsung cells, ans also Liitokala 18650B copies, and in both cases the Current was the same.
My photos are quite crappy (sorry about that), but you can see that there are two low value resistors in parralel, and they must be the sensing elements for current regulation.
I hope somebody could also examine this driver, with a better photo-apparat, and confirm this theory. The FET is in the negative side, between the LED and the Battery-, and the Sense resistors connect the Driver spring and LED+. If these resistors are not for sensing, I do not see why they even exist.