Aerobic and Anaerobic metabolisms are not mutually exclusive but, rather, synergistic
The great misunderstanding of the "fat burning zone" myth is the idea that fat burning decreases as intensity increases. The percentage decreases, but that's as a proportion of an increased total. The absolute numbers do not decline.
[myth is the idea that fat burning decreases as intensity increases.]
The relationship between anaerobic high intensity and muscle glycogen isn’t a myth. Nor is the difference between the metabolism of fast and slow twitch muscle fibers.
A 100m sprinter is burning glycogen during the sprint, not fat.
It’s less commonly recognized that “waste” products of high-intensity anaerobic effort (lactic acid for example) are metabolized aerobically.
Thus, the more developed the Aerobic physiology is, the great the Anaerobic effort that can be maintained over a longer period of time.
The associated fitness can be measured by observing recovery HR - as Anaerobic byproducts are metabolized, oxygen debt is reduced, and the HR is reduced to below the Anaerobic Threshold.