After referring to the book again "Aikido Shugyo", this is what he wrote in a nutshell...
It is all about timing, when uke has fully extended himself, you enter strongly at the moment he/she is fully extended to repel/bounce him of your body. It is easy to say this but to do it... is another ball game altogether.
It is also incomplete. There is not any point in the attack where it is not vulnerable from the tangent or normal to its line of action. The point of full extension is merely one point of vulnerability in that progression where momentum reverses, and there are any given number prior to and after that in the recovery phase. This is what O Sensei referred to when he specifically denied the principle of sente in aikido, of which timing is a part.
I think Shioda may have meant something broader than you are taking from it. He certainly does not wait for full extension in HIS randori.
Moreover, the firm "bouncing off" "rooted kokyu is only one half of the equation -- ten no kokyu as well as chi no kokyu make up the whole of kokyu ryoku that Shioda wrote about, and his demonstrations clearly demonstrate.