. . . say he had done so, and IBM very much did not want to buy their operating system from him because that was a serious violation of IBM's sacred culture.

While there is no definitive report I know of (the best I've seen is the book "Big Blues") the most solid evidence is that he was in Japan discussing operating system deals and the airplane story was made up by Gates and/or IBM for their own purposes.

In any case, IBM met with his wife, who was the proper company officer for them to meet with, but IBM didn't do business with women in those days. In any case they presented their contract and it was rejected by DR's lawyers who told Mrs. Kildall she needed to negotiate a number of one sided points.

IBM presented the same contract to Bill Gates, who had no product and so no real risk, so he signed it figuring he could renegotiate later if he was able to purchase a product somewhere.

In the end, Microsoft didn't have the ability to finish MS DOS, so IBM had to do a lot of it for them to get a working operating system, probably why DOS was relatively stable.