Perhaps it would be better to say that Ki is something limited to physical expression. Or at least, restricted to the boundaries of the flesh and bone in this particular instance because O'sensei was still limited to the weaknesses of the human body, despite his skill.
Well...I'm not one of them, but there are those who would disagree with you. Personally, I believe that there may exist phenomena that we can't measure (or can't measure yet), and can't see direct and obvious physical expression of, but we're on shaky ground when we assert the existence of such phenomena based on faith or belief or wishing it to be true, rather than empirical evidence. Once upon a time human beings didn't understand how the tides worked, but before long people observed that they seemed to have some relationship with the moon. At that point I'm sure many legends arose about some mystical power, a god calling to the seas, something like that. The legends correlated with observable facts, but they were not evidence-based and they were, in fact, fiction. I tend to think that if ki exists, it is much the same. It has not been measured, it currently can't be measured, and except for those who use "ki" as a catchall term to refer to various physical phenomena, the stories and assertions that seek to explain ki are like the stories about the moon god calling to the ocean. But to say that it's "something limited to physical expression", I think, we need to remember that for all practical purposes, "physical expression" is limited by our ability to detect and measure it. What we can detect and measure now is not necessarily what we will detect and measure years from now, but that doesn't mean it doesn't exist.