The way I interpret it, it's 18 hours full functionality with all the stuff listed in the battery test (IE, workout, music over bluetooth, app use and checking the time every 12 minutes and so on, which is probably more hardcore useage than most would demand of their watch, I would think), then 72 hours of power reserve on top of that.
Wouldn't make much sense to brag about 72 hours power reserve if it meant turning off all the stuff that makes an apple watch worth having... 😛