Plenty of people make that claim, just search carbohydrate loading. The scientific reasoning is very simple. Aerobic exercise performance is limited by the rate

Plenty of people make that claim, just search carbohydrate loading. The scientific reasoning is very simple. Aerobic exercise performance is limited by the rate of oxygen uptake, turning fats into usable energy takes considerably more oxygen than it does for CHO. You can in a single race/session exhaust your glycogen stores, but the supply of fats is essentially limitless. So for ideal performance you want to maximize your glycogen stores before an event, and for long ones, take additional CHO during, though rate of consumption is usually significantly higher than replenishment through digestion if you are going at all fast. That point when you have exhausted your glycogen stores is what they call bonking. Now you can fine-tune your low carb diet to hopefully have enough glycogen at any point to sustain you through an exercise session (more realistic for running than cycling) but everyone else just eats a high-carb diet.