Hill Sprints are a form of strength training and are not Z1 training. They do not really fit into a zone category. Intensity zones are used to designate variations AEROBIC work load so you can control the intend training effect. Hill Sprints are too short (and by using a very long rest) have minimal aerobic energy contribution. As described they are relay heavily on stored ATP and to a lesser extent on the anaerobic energy production system.
If done maximally and in a high enough volume that will reduce muscle glycogen stores. I would not count on them to replace a fasted Z1-2 longer duration effort though.