Surprised about no replies. Is the interesting discussion happening on some other forum?
Regarding the recommendation against solo practice:
So we know O-sensei did lots of solo practice, many people saw him. (besides the heavy spear work, lodging and un-lodging the spear into trees, we also have the empty hand "tree pushing" exercises as photographed.. which was called kokyu training.
So, it could be that his sentiment here is referring to a distinction between kokyu ryoku (needs solo practice) and aiki (needs partner practice). Not saying I entitrely agree, just saying that putative distinction could account for this discrepancy as we have seen it worded thus far.
Another possibility is much deeper culturally. Sunadomari Kanshu said that within the Oomoto circles, they called Japan's efforts in WWII a "losing war." This had to do with the Japanese government violently suppressing the Oomoto religion-- the believers saw this representing exactly what would happen to Japan in turn during the war. Could it be that Ueshiba and the rest of the cult members felt spited by the Japanese government, and had no faith in the war effort? In such a situation, even if commanded to go and instruct at imperialist overseas universities or teach directly at military institutions, would O-sensei hold back information that would help progress in budo, or even say the opposite of what he believed would help? It was a crazy time, not just for Japan and the world, but for O-sensei as an individual.