This operation was obviously planned for months, probably years, cost huge amounts of money and held enormous risk. You don't then go and stick four idiots on the plane who are good at MS FltSim! They probably weren't airline pilots, more likely extremists with either some ex-military experience, or maybe they were put through a flying school with some time (type rating?) on 757/767.
I don't think it's a coincidence that all four aircraft were the same (flight deck). Pity United didn't stick an Airbus or something on that morning - that would have surprised them! It's quite possible that they could have done all their flight training in the US - there must be 757 sim in Florida for hire.
Just imagine it - "so, we've got five minutes left at the end of the session, anything you guys wanna do?" "Can we try crashing into the side of the world trade centre?" "er, I guess"
Chilling.