I have an uncle who just turned 80 last month. He spent 30 years as a cop, and for many of those years worked off-duty as a gravedigger, both to earn extra money (California cops weren't paid as well in those days) and to stay in good physical condition. They dug the graves with pick and shovel instead of backhoes or other modern equipment. Harry had a grip that was intensely powerful and was able to burst the seams of a beer can BEFORE beer was in aluminum cans. There are simply some people who have incredible grip strength naturally and others who develop it through their work or activities. I don't doubt O Sensei's reported grip strength and I'm not surprised that a man who farmed for much of his life and wielded swords and bokken most of his life would develop an extremely powerful grip. The IS folks may burn me to the ground, but it seems like a natural kind of adaptation to me.
Never discount 'farmer' strength. My grandpa used to string barbed wire fences, miles of the stuff, using only a single pair of combo pliers/wire cutters. All day every day both hands for weeks at a time year in year out. Nobody would shake hands with him. The only person I know that even came close was my grandma who would macrame all day for years and she would put a pinch on a wayward youth that would literally drop them in their tracks.
Grueling, backbreaking, hardcore physical labor 12-14 hours a day for years on end ... pretty unusual to find these days. Wonder why the youth so readily flocked to the big city?
Even if IS is a different kind of strength imagine adding that on top of the hardened muscular foundation of a 'farmer' ... scary.