The Claim: Cardio on an empty stomach in the morning causes you to burn more fat than glycogen
After an overnight fast (sleeping) you will be burning a higher percentage of fact vs glycogen simply because you haven’t eaten in a while. By the time you wake up your body will have burned a significant amount of the glycogen stored in your liver and will be relying on an increasing amount of fat for fuel.
Doing cardio first thing in the morning on an empty stomach doesn’t make your body burn any more calories than it would have at any other point of the day. Also it doesn’t really change the amount of fat that would be used for that particular exercise.
The time of day is not what determines the fuel used for an activity, the intensity does.
For example, while you are asleep your body burns a high percentage of fat. On the other hand a sprint intervals require predominantly glycogen for fuel.
The time of day of your cardio workout is irrelevant to fat burning as your total calories in vs out will determine if you can reduce your fat stores.