I just verified that with mine...the crenelated tailcap does require one to insert their thumb or finger directly into the tailcap, instead of being able to activate or deactivate the flashlight with a lateral thumb or finger motion.
I'm not familiar with grips to use a flashlight with a gun, so please do not ask.
But these crenelations do preclude the use of the E2D with a gun using the gripping techniques I've seen photographs of.