From what I could find, that is indeed the first documented reference for it.
Joan's daughter Christina Crawford was credited with writing the memoir, which was turned into a movie. Could she have been at the meeting? Was it told to her by someone who attended the board room meeting? Was it something Joan said regularly? Or was it something a ghost writer made up for color? Could Christina have made it up?
This link says Joan never said the rodeo line but doesn't give any explanation for this conclusion.
At this point, it is a bit of a mystery. But it gives an interesting look at what a woman had to put up with serving on a male dominated board of directors (Pepsi in this case).
Warning, do not open the video in a public place. It contains vulgar language.