Cardio on an empty stomach can be a benefit if glycogen stores are low enough where you body can tap its fat reserves...
People don't seem to remember that the body can store vast amounts of glycogen in the liver/muscles/bloodstream etc...
If your storing 1200 grams+ of carbs in your body...And you wake up in the morning, and do 2 hours of cardio on an empty stomach...Guess what it's very likely that you burn only tiny amounts of fat..Your body doesn't like to use fat for energy production when there is plenty of circulating glucose in your system...