Near the end of the article, which (although yes it does analyze things) attempts to excuse them, ahem excuse me, but the hijacker/bombers/terrorists were not poor.
I believe you are right - what I think the author might have been trying to get across was that they originally came from historically poorer tribes near the Saudi-Yemen border (as did OBL's poppa).
But I think you are quite right in pointing out that the 9/11 human bullets were all from middle-class families & were all well educated.