I believe this is the paper: https://jissn.biomedcentral.com/track/pdf/10.1186/s12970-020...
Seems as though we're talking about MFO (maximal fat oxidization) of 10.7% in the morning and 29% in the afternoon. Interestingly you see an increase in VO2max as well - which I believe is essentially blood oxygen.
A few things I'll point out though:
1. This is just 15 men, aged 32 +/- 7 years. Doesn't quite seem like enough people to definitively draw a conclusion.
2. They are described as 'caffeine-naïve' (which I assume means they do not normally consume caffeine) - so it's unclear if this effect would be the same to somebody who already regularly drinks caffeine.