In other words, your ability to increase your speed beyond a purely aerobic intensity isn't really based on your anaerobic system, but rather on your aerobic system. (philmaffetone.com)
In real world terms it defined that fat was only relegated to low and mid-level intensity of exercise and did not play a role at higher aerobic intensities as seen here: (vespapower.com)
As you can see from the graph, the high - carb cohort data falls consistently in line with the current published literature, but, with the low - carb cohort, not only does the fat utilization shift into the realm of higher aerobic intensity (the realm which is regarded as the «sweet spot» for endurance athlete performance) it does so at a much higher rate of fat utilization. (vespapower.com)