You would have to shave out 1mm from the entire wall of the battery compartment. Not just the back slot or the door, the compartment is physically too tight to squeeze a 21700 in (the black side walls are too close together, not the front or back where the panels are but the molded section above the bottle and the side that has battery orientation markings).
Correct. There's a You Tube video on modifying the Pulse 80 to take 21700s. It requires much more than merely modifying the battery door. Not worth it IMO. If the 21700 is important better getting the Pulse X.