To me, this whole exercise while fasting idea seems pretty "out there".
But I'm willing to be convinced, what does the science say?
A quick search found this review article,
Exercise Training and Fasting: Current Insights.
The final sentence of the abstract:
Our review of the literature suggests that there is little evidence to support the notion of endurance training and fasting-mediated increases in fat oxidation, and we recommend that endurance athletes should avoid high intensity training while fasting.