that the primary thing that Ueshiba was training in order to manifest "aiki" is a training that is shared between his art and other arts, some of them Chinese. The reason for this homology is that the training methodology was in fact developed long ago (partially in China, partially in somewhere like India where the training culture would have come from before making its way to China).
There is actually a fine point here. You posit that there was a training methodology
to develop aiki skills that was the same thing as was developed long ago and came from China and India. That would mean that the things that Takeda taught and Ueshiba trained and taught were the same things as these Chinese and Indian masters from further back taught.
Is that so? Can this be shown?
One concept that I hear of with regard to Chinese internal arts is solo training. That would seem to be a consistent methodology. If that is a hallmark of Chinese training there is a problem because Takeda didn't teach solo training to any of his students.