[1] https://care.diabetesjournals.org/content/25/2/364
In [1], it is shown that caffeine decreases insulin sensitivity in cells. E.g., muscle cells use less insulin.
[2] https://academic.oup.com/endo/article/142/6/2702/2989741
In [2], it is shown that insulin inhibits mitochondrial oxidation of fatty acids.
These two findings support the finding in OP - if muscle cells use less insulin due to higher caffeine levels, they use more fatty acids oxidation.