My understanding is that the plane was No. 1 to take off (not just boarded and doors shut).
That's close to take-off, for sure. but I'm also sure that the Feds could have had the plane diverted, once it was in the air, too. It wouldn't take much to send a couple of military planes after it and direct it back to an airport to "disgorge" the terrorist.
So, even though that sounds fairly dramatic (and it probably is...), there was still "measures" that they could have taken to "get him" in any case...
The pilot most likely would have turned around if he was not too far out over the Atlantic. We may not have extradition with Dubai, but the airline certainly wants to do business in America.