Actually, seth, you're using pre-Crisis logic. Post-Crisis on Infinite Earths, Kryptonite is not as effective against Superman. It would take days of green k exposure to kill Superman, but the immediate effects of it wouldn't be much beyond making Superman nauseous and not as overwhelmingly powerful. Not ot mention, he was out of the Earth's atmosphere when he threw the island, and it can be argued that he was absorbing much more sunlight than he would normally be.
I would have preferred an actual fight to island throwing, myself, but que sera.