Holding a knife in a grip that isn't in line with your forearm is by definition going to leave you with a shorter ranger. Go ahead and hold a knife in your hand in a regular grip right now, put the point against a wall, and stretch your arm and body until you are standing as far away from the wall as humanly possible without taking the point offline. Now, without changing where you are standing, go ahead and put the knife in a reverse grip. You won't reach.
The icepick is not a 'defensive' grip, one does not truly 'defend' with the blade of a knife in a knife fight. It's for getting in puncture angles that a forward grip cannot conventionally achieve, like straight downward into the shoulder, or up into the offside armpit. It's for either murder or armor penetration, and doesn't make sense in a modern knife fight context. Go ahead and watch the rest of that guy's videos on the same subject and he elaborates on this. It's also an instinctual thing related to how we naturally would use our fists as cudgels.
In the meantime, more ass.