The kata does not seem to have been taught by Kyan Chotoku. I believe the principle instructor of Shinabuku Tatsuo. However, today, many of Kyan’s students student’s branches use the form, but, as I understand it, their versions of Nihanchi Kata all came from different sources. Obviously those that adopted it felt it is important
Perhaps this is a proof that Motobu Choki was an instructor of Isshinyyu’s founder and the source behind Isshinryu;s version of Nihanchi Kata.
One of the more perplexing issues with this theory is Motobu Choki, in his early book on Karate, was against the idea of the inward toed stance writing that it was too weak. Some Isshinryu lineages use this stance,. Just because of those writings its not to say in time his opinion changed. as time passed
That makes it difficult to accept Motobu is the source. I just found this on Youtube and am linking you to it. It is Motobu no Naihanchi Shodan - Motobu, the son of Motovu Choki. Note how similar to how Isshinryu Nihanchi Kata begins. I note that on the Japanese video of his father’s art Motobu Chōsei also shows a class with Naihanchi Kata beginning on the right.
I suggest it implies that the direction didn't matter in the Motobu Choki tradition, just the practice of the kata. So perhaps this is sort of proof for Motobu. Sort of makes sense for the kata is covers the same ground no matter which way you start.
So perhaps he was inspired by Motobu's training, and then Shimabutu Tatsuo used his own ideas to re-craft the form.