Right, but the question is really do you have to exercise in your maximal fat burning zone to get better at maximally burning fat. From my perspective, if I'm not maxing out the rate at which I'm getting better at burning fat, then I've left something on the table.

Some sources suggest that you are not maximally using your fat burning energy pathways at higher energy intensities. Like this study, a clip of which is shown below: https://www.ncbi.nlm.nih.gov/pmc/art...B1%2C%2022%5D..

This kind of suggests that the top of Z2 is where you begin to sub-optimally engage your Z2 / fat oxidation energy pathway.