I also have a Charge 2. I've found the calorie count is consistantly five to ten percent high. Many people have found the same thing.
Fat burn is by definition aerobic exercise. The question is the intensity, which can be determined by the calories per minute burned. There is a gray area between fat burn and peak that is a mixture of fat burn and carbohydrate burn. In this area, it carb consumption rises from about 30% of calories burned to 70% calories burned. As it rises, the exercise becomes more anaerobic.
Fitbit has a magical formula that is not soley based on heart rate. It also measures steps taken and the speed of the steps. I recommend trusting it is accurate enough.
Fatigue is not necessarily a good indicator of calorie burn. For instance, when I lift weights, I only burn about four calories a minute even though I don't take any rest breaks. When I walk or jog, I burn about nine calories per minute with less fatigue. I stay in fat burn almost the whole time.
If you'd like a comparison of the aerobic benefit of different exercises, you can find a list here.
