Do you have a citation for that quote? My understanding is that it was very rare for O Sensei to take ukemi.
He said that in an interview for a Russian group. They translated their questions into English, I translated into Japanese, then Isoyama Sensei responded in Japanese, which I translated back into English and that got translated into Russian. There is a video of it out there somewhere, but I'm afraid I don't have the link.
This is part of the transcript I gave them so that they could make subtitles:
<Carl> How long did you practice with Osensei for?
<Isoyama Shihan> It was 1949 when I entered this dojo and Osensei passed away in 1969, so it was about twenty years. But I was only able to practice with the founder (not only taking ukemi but being able to do techniques on him) until about ten years after I started. In those days when I first began, there weren't so many students so everyone who came to practice would always receive Osensei's techniques once each, on both left and right sides. Particularly with regards to techniques there was little explanation but when a technique was done on another person we looked with wide eyes so as to really see how Osensei would make the technique, how he would use power and so forth.