As intensity of exercise increases, the proportion of energy derived from fat decreases, and the proportion from carbohydrate increases. However, where the energy comes from during an exercise session doesn't affect net energy balance, only total calories burned does that.
The effects of EPOC mean that, given a limited amount of time, HIIT can lead to greater total caloric expenditure. However, someone is more able to increase the amount of moderate intensity exercise they do, and recover, than increase the amount of high intensity exercise they do, and recover.