There is much more to the fire/water trope and it is worth reading. The significance of this quote is twofold on this point, however.
First, O Sensei had no doubts that his metaphorical (or alchemical) understanding of the operation of the art could eventually be described in Western scientific terms. Second, he firmly believed that it ought to be done by someone following him.
I look to go where he pointed. I do not dispute that he pointed other ways also. Kokyu expands in all directions simultaneously, so there is no conflict or contradiciton in that, at all. I just hope to find somebody farther along to make this particular path a little clearer.
Reading your posts, I usually have one over-riding area of concern. When you talk about Ueshiba, or quote him, you translate/define/whatever his writings in a very definitive way as if you know 100% what they mean.
As with the above, "he had no doubts" and "he firmly believed". I find it hard to understand how you can know that he had no doubts or where his beliefs were firm. Maybe it's just a writing style that you use and I'm reading more into it than what's there, but ... as I said, it comes across to me as if you understand perfectly what Ueshiba said and wrote.