A thought on this:
Christopher Li wrote:
Takuma Hisa ...gives there the same exact explanation (word for word)...
"Strike while exercising the technique of the unity of opposites (陰陽合致) in the hand-sword."
So if I am thinking clearly, one of these is likely:
1. Both the Ueshiba documents and the Hisa book are quoting an existing Daito-ryu document;
2. Both are quoting a received oral teaching from Takeda;
3. Hisa is quoting Ueshiba.
I'm absolutely not surprised they are saying similar things, I just thought it was "striking" that it was verbatim.