From what I just read, it sounds to me like you're saying the biggest difference between IP/IS movement and athletic movement, is not so different in the way you are using the body, but instead in the way you are thinking about and/or feeling your body.
Is this correct?
No, that's not what I am trying to say. It is this different way of thinking and feeling that leads to a fundamentally different usage of the body.
Now it may not be totally fundamentally different from a physiological perspective (because both forms of movement are done with a human body although very differently developed), but until physiology can explain both athletic and IP/IS movement, there's no way of knowing.