That capacity is very impressive, although I have to ask if it has been tested. The best 26650 cells are topping out at 6000mah in tests and cells that claim more usually test significantly lower. If a 21700 battery opens up new opportunities for technological advancements in cells that can't be applied to the 18650 format, then I suppose I would have to reconsider.NPL, Acebeam has a 21700 battery @ 5100mAh (with 20A discharge capability). I would think Zebralight could source their own cell. This is very comparable, and in a smaller form factor, to a 26650. I'm all for the 21700 format myself. It will be interesting to see this light once it is released.
With technology evolution usually translating to better battery capacity, more efficient drivers and better LEDs, the one factor that hopefully remains constant is the total size of the flashlight. I would personally prefer to keep the lights as compact as possible and let other technological advancements give me better runtimes, more lumens, throw, etc. as oppose to jumping to a larger battery size. All a matter of personal opinion though.