Guten tag Bernd,
Just because your sensei said something doesn't make it true, particularly one with a hero-worshiping bias such as your sensei. Also, define "outstandingly strong". He may have been so, for his age and his time, however, all people are subject to the ravages of age and deterioration, particularly senior citizens. (besides what is your sensei's frame of reference?)
I live in a US college town with some of the finest athletes in the world. Trust me, they're "outstandingly strong" (20+ reps of 400lbs bench press, sub 4.40s 40meter dash, etc....) literally Olympic caliber athletes. None of us can even begin to relate to their physical skills and abilities. Osensei was no where near this level of pure strength and fitness EVER, and if so where are the unbiased records? (I've seen all same pictures you have and I don's see anything special.)
Sadly, 80-year-old, 5' tall, 130lbs men never have been, and never will be capable of such scientifically measurable, objective feats or strength and fitness, regardless of one's level of "ki mastery". Then again, I can't prove the negative and you haven't proven the affirmative (personal, biased anecdotes are not evidence). Aside from all that, it's just good old Human Anatomy and Physiology (I'll be happy to debate that with you as well), as we age we deteriorate :/
There's a reason you don't see +50 year old boxers, or tournament fighters, they're (relatively) old, slow and weak. However, these limitations can be overcome with proper technique, as I initially started, but only with proper technique. All things being equal, the younger, faster, fitter fighter wins.
It's like golf, while the young guns generally prevail, there are a few old masters who have refined their stroke over the years to compensate for their deteriorating physical attributes to still remain competitive, but this is generally the exception rather than the rule. (Oddly, Tiger Woods the child phenom, is now having to assume this role of the veteran player who relies more on technique than raw power. Proof positive that it can, and does, happen to the best of 'em.)
Osensei's greatest fault is that he relied far too much on the opinions of others and himself to promote his art, rather than with consistent, unbiased, certifiable achievements. This is also why Aikido, in general, tends to get a bad rap as being a "weak" art. I'm no fan of the Gracie family one way or the other, but they did take on all comers at all times without exception, and you've gotta give them credit for that. They also recorded as many matches as possible for authentication. They put their money where their mouth is, which is more than I can say for many in the Aikido community. It need not be a "competition" for there to be a genuine test of skill. After all, we test nuclear reactors, without turning them into nuclear bombs.
Although I love the art of Aikido and acknowledge Osensei's ingeniousness and great skill, I laugh out loud at all the old "I'd love to give the emperor a demonstration of my art, but it's just too dang lethal" excuses. Yeah riiiiiiiight, and I got a bridge to sell ya
Also, did I mention, I just wrote a symphony ten times better than Beethoven's ninth, but its just too good to let anyone hear. It might kill them. LOL!!!!!
I've also never seen a vid where Osensei was genuinely challenged by a decent, unbiased competitor (lumbering 1950's era , US "juodoka" notwithstanding). After all, where are the leg kicks, take down attempts, decent boxing with cross, jabs and upper cuts? You haven't seen one either, because they don't exist.
For better, or worse, the old man was expert a ducking a challenge all while claiming innate superiority on both a physical and "spiritual" plane (that is if you count invading Manchuria on a ridiculous and rather cruel religious crusade a "spiritual act"). Maybe he was an athletic phenomenon beyond all reckoning, maybe he wasn't, but without concrete, unbiased proof one must withhold judgment on such speculations, rather than relying on others opinions.
"That which can be asserted without evidence, can be dismissed without evidence." -Christopher Hitchens