He did not go outside of Aikido to bring the skills back into Aikido...
What drives me nuts in these discussions is so often people get in to "binary" discussions. Yes he did, no he didn't...
There is *no* doubt that Tohei trained with Nakamura who himself was considered the "father" of yoga in Japan. There is no doubt Tohei was profoundly affected by this training.
My understanding of the issue is that his training with Nakamura helped Tohei better understand what he was experiencing with Ueshiba. And that allowed Tohei to develop his systematic approach to teaching which in turn became "mainstream" aikido since Tohei was soon Chief Instructor. So there was an influence. I wouldn't say he had nothing then went outside to bring ki back. Nor would I say he already had it all and he just did some yoga on the side.
Now the question that remains is whether what he was doing/teaching/formulating was the absolute end all of aikido or the *only* way to go about transmitting the skillset. I think few would claim such a thing. I train in a lineage that goes directly to Tohei-sensei. I like/enjoy/train/teach much of the same stuff. However, time spent with others has greatly informed my understanding of what we learned from Tohei, from our founder, Kobayashi-sensei, and then from his senior students now carrying on what he taught. I have spent time with Mike, Dan, Toby, even Aaron Clark via an Aikiweb seminar. I learned much in all of them, adding things to my "to do" list each time. Stuff I try to do daily in addition to what I already do. And yet when I train and teach I don't think I'm doing anything outside the realm of Aikido. I just find there are new ways to improve, new insights, clarifications, etc.
Anyway, shades of grey here.