I wonder whether the boy was filled with rage and it was the intensity of his attack that overwhelmed the 6th dan? I wonder whether psychologically the 6th dan couldn't bring himself to fight a teenager even in self-defence? Whatever the reason this is a poor indictment of Aikido.
I'm sure there are many more cases of Aikido being used effectively in self-defence. But it's an important reminder that training with like minded people who respect one another in the dojo can be deceptive if we don't respect our opponent in a real life attack, a little like a boxer underestimating their opponent as they are too focussed on winning the world title further into their career. Achieving the more moral and philosophical state of mind that O'Sensei discovered and encouraged and the transcendancy of strength in technique is if you like many an Aikidoka's world title. But we must not overlook or underestimate the necessity of making Aikido work in reality while on the journey.
Just because someone knows a martial art doesn't mean they know how to fight.