It should also be remembered many of O'Sensei's students were experienced in other martial arts. Nowadays students rarely have any other experience and do not understand the dedication, which is needed.
As I implied when I made the previous posting, Morihiro Saito made it to san-dan in three years without much extensive experience in the martial arts (he started with Ueshiba when he was eighteen).
Even today, I've seen people in Japan make it to san-dan in four years or less. It's not that unusual, and they didn't train that hard or have any special experience.
All that should be taken into consideration when thinking about what Ueshiba said about ki no nagare starting at san-dan.