I didn't know that... but it is actually a good grading system.
In classical jujutsu schools, there are 3 levels of techniques (not grades):
Shoden - based on the form of the technique as is
Chuden - based on the principle of the technique
Okuden - based on being able to use ki within the technique
I guess we could correlate this with Tohei sensei's grading on ki:
Shoden - understanding unified mind and body through posture or form
Chuden - understanding Ki through unified mind and body
Okuden - understanding Ki
Taijutsu is easier to grade, it could be based on form/technique only (physical side). But for Ki development it's not so easy.
Like the classical jujustsu, Aikido techniques also exist in those types of levels. And knowing which level one exist in, it's actually very subjective of the sensei to determine.
One could be a shodan but still exist in the shoden level. This could mean that the shodan is mostly technique based. Many aikidoka (even yudansha) exist on this level.
With ki no kenkyukai (ki society), not shin-shin-toitsu aikido, technique is not actually needed. The techniques are usually used as a way to test the ki. So in this society, one could understand ki very well but know very little about aikido techniques.
Since ki no kenkyukai is not actually aikido, it can't be graded through the kyu-dan system. The only system that could be applied, is based on one's understanding of ki.
Well... I hope someone form Ki no Kenkyukai or someone from Shin-Shin-toitsu Aikido, or any of Tohei Sensei's student could verify my analysis...