That chart supports what I wrote. At about 65% fat usage drops and carbohydrate usage shoots up like a rocket. The study I read only tested three points: 65%, something less than 65%, and something greater than 65% (IIRC, 80%).
The Training Peaks article references work done by Len Kravitz. He has a blog I stumbled across several years ago that has good articles about exercise and links to other works. Worth a look.
https://www.unm.edu/~lkravitz/Pages/articles.html