None of those men have talked about the importance of koshi? I'd find that very hard to believe.
Edit: And I know that Buck wrote "pelvis" and not "koshi", but the term "koshi" includes the pelvis.
I haven't encountered it. Not in any of Ueshiba's doukas, not in Shioda's "Dynamic Aikido", nor did any of my former Aikido teachers say anything important about the pelvis. I didn't mean to suggest that these men 'didn't' talk about it, it's just not prevalent in public literature (excuse my poor choice of words). It's been a while since I read Tohei's books on "Ki" but for the most part he, and Shioda, and Ueshiba talked a lot about 'breath' and 'breath training'. Usng the pelvis is just not a 'fundamental' or 'well known' concept as Buck describes it.
Lately, since the 'internal strength' thing is raging now in the American aiki community, discussions on the pelvis have been popping up, but even then, ideas on how to use it and how it affects the body structure, the fascia, etc. are still, I believe, underdeveloped and unclear, contrary to what Buck ignorantly supposes.