They don't have to be very bright. Just vicious enough to want to kill total strangers drinking coffee at their desks some September morning...
They obviously thought they were in touch with somebody who was going to plan things for them, and this is what AQ has been looking for: domestic people who are dumb enough and violent enough to let AQ use them for operations here. Fortunately, dumb also means more likely to get caught (like the shoebomber, Muslim convert Richard Reid, whose IQ was probably only a little higher than that of his shoes).
They couldn't even get a digital camera on their own to take pictures of the local FBI building - the FBI gave them a camera. They had to get their "boots" for their "war" from the FBI agent (yes, the FBI gave them boots). They don't seem smart enough to even form an intent to harm. Richard Reid is a genius compared to these clowns. This "plan" was going nowhere. That said, I am glad we got them.