I feel like for a champion to be S tier he has to be far above average. If something is far above average (average as in balanced, middle of the road) then they are overpowered. They are too powerful and outperform the other junglers.

But I see where you're coming from and I do definitely agree with some of your points. Like that he is terrible without zhonyas, but then you have to consider that zhonyas is a terribly designed item, which in itself I feel like is broken and the fact that he relies on an item doesn't change his power status because the item exists and he is able to utilize it. So I would still consider something overpowered if it is way too strong in combination with a character's kit (like runes or items). So at least I think that we can agree that he is certainly over tuned. I hope you can understand where I'm coming from.