That same company also purchased over 100,000 shares of United on Sept 10th... why do that if they knew what was coming?
The reason they shorted the company is because of a Sept 5th Reuters report, where analysts expected a further decline in travel companies. You may remember that all transportation got rocked that summer, including airlines and cruise ship companies. News came out, they shorted, share price dropped, then they bought back in. That means that company actually expected the price to rise, which indicates they didn't know 9/11 was coming.
Anything else? Please bring up thermite next, that's my favorite one to disprove