"A measure of abnormal long put volume was also examined and seen to be at abnormally high levels in the days leading up to the attacks. Consequently, the paper concludes that there is evidence of unusual option market activity in the days leading up to September 11 that is consistent with investors trading on advance knowledge of the attacks".[67] —Allen M. Poteshman, The Journal of Business
Yeah it only takes a small number of investors acting weirdly to trigger an abnormal reaction by others. So it could be a coincidence or it could be one or two Saudis knew what was up. Osama had a big family connected with big Saudi businesses.
There's no chance a large number of investors knew about the attacks. It would get out.
If a large number of investors knew about it and took advantage to make a ton of money, at least one of them would have stupidly bragged about it, and we would know.