Beat me to it.
You store glycogen in your muscles and liver. A world class cross country skier (the elite of the elite in aerobic power) can burn through almost all of their glycogen in about 60-75mins. So it would take the average athlete 2.5+ hrs to burn through theirs and rely almost entirely on fat metabolism. Even if you could do that, you would probably bonk (complete collapse where you can barely remember your name) before you got to a state where you were relying almost entirely on fats.
The reality is that you will use some glycogen, some lactate, some fats, and very little (to none) protein, at all times. If you reduce carbs, and minimize carb intake during aerobic efforts, you'll use more fats. Unless you are highly trained or starve yourself, it's unlikely you will go into a glycogen deficient state. The reason this studies show little to no benefit is that it's really hard to use up the stored glycogen.