That is true, the parks were not Actually "closed" for 9/11, they did open that morning, (as the events were still unfolding) they in fact closed early that day and opened on 9/12. So 9/11 does not count as a park closed day, it was an early close... as was the day before Hurricane Erin, the day before Hurricane Flloyd, the day before Hurricane Charley, Frances and Jeanne.
As you can see it is not normal and this year has been the most closures that the parks have seen in their history.