Fat vs glycogen burning at different intensities is highly variable between individuals, and also depends on an individuals fitness. The fitter you are the more power you can make without using much glycogen.
Even when riding at higher intensities when you are using glycogen and fat for energy, the kcal to replenish the glycogen after the ride has to come from somewhere. If you don't over eat, some of it comes from stored fat. So an hours intense ride burns more fat than an hours ride in the "fat burning zone".
HRM based calorie counts are completely useless and often read very high. They are for entertainment value only.
If you held a UCI elite license you know how to ride. Just go do it and eat healthy. You'll lose the fat and get fit and have fun.
"entertainment value only".. LOL.. so true.