High intensity cardio is not "cardio" as we think of it. It is simply another form of anaerobic training. Fat requires far more oxygen to be oxidized than does glucose. When oxygen is limited as it is when exercise intensity is incresed(generally above 70%VO2max) there isn't enough oxygen to suport the oxidation of fat. Glucose on the other hand requires very little oxygen to be oxidized thus it can be converted into ATP with very little oxygen present as is the case when exercise intensity increases.
For example. It takes 6 molecules of oxygen to oxidize one glucose molecule. It takes 26 molecules of oxygen to oxidize on stearic acid molecule. For most people fat utilization is limited above 60%VO2max. Breaking down fat into ATP is a slow process. When exercise intensity is high energy demands are imediate and need to be met very quickly. Glucose can be oxidized quickly whereas fat can't. This is advantageous during high intensity training so the prefered fuel source is glucose(muscle glycogen).
Proponents of HI cardio training are simply proponents of anaerobic training. This form of exercise utilizes glucose/glycogen just as weight training does. In this envoronment oxygen cannot be supplied fast enough to oxidize fat so glucose is used. Only when ample oxygen is present will fat be burned.