IMO, it is the size of the aerobic engine that matters. For a UCI grand tour pro, they can sit in the peloton in recovery or low z2. At that aerobic level, they are not burning much glycogen. And they ride a lot at that power level because if they rode a lot at higher power, they would run out of glycogen. We all have the about the same amount of glycogen storage.
I think it is true that bonk / fasting rides can improve fat utilization to an extent but not to turn a 300 watt FPT racer into a 400 watt racer. Professional athletes in general are genetic freaks and have the combination of genetic attributes that coupled with training allows them to compete as a professional in sports. If hard training was the answer to achieving a very high level of proficiency in sport, we would all be olympians.