Intramuscular fat, stored in each muscle cell, is depleted with endurance exercise, just as glycogen is. This fat can only be replenished quickly if fat is consumed.
If intramuscular fat is not replenished, fat oxidation rates remain unchanged, but the fat is pulled out of the blood…which gets its fats from adipose tissue. This seems optimal for people that exercise to improve body composition.
For athletic performance, it is important to replenish these fuels. The researchers found no difference in 20k time trials, but at that distance glycogen is not completely depleted. Replenishing intramuscular fat would seem to be most important for distances where muscular glycogen is depleted…marathon and beyond.