If you wait enough days between HIIT, glycogen stores get refilled with the glucose made from gluconeogenesis. Dr. Eades mentioned some research that showed that the first place "new glucose" went, was to glycogen.

Also, the amount of glycogen an athlete can store, you can train for it so you have more glycogen storage for explosive exercise. It's a training adaptation. Plus people vary according to their mix of muscle type and glycogen storage to begin with.

HIIT every day would be just as glucose-demanding as long hard cardio every day. I seem to remember most HIIT advocates also advocate days between sessions. Something that appealed to me