I agree.
Frankly I'd think it would be difficult for them to acquire and then to hide enough nukes from radiation monitors etc to deliver a crippling blow to the US ... if we found even one the game would probably be up for their plans. One or two nukes possibly, but not the dozens that would be needed for a truely crippling blow.
Infectious biologicals can be much more deadly than a couple of nukes and much more easily hidden, since they're not possible to detect with external sensors until they're actually released. Plus you might only need a few dissemination points if the agent is infectious enough. That's especially true if you aren't too concerned about losing a few hundred million of your own people in the blowback from the spreading infection and any retaliation that might take place. And I don't think that they'd think that was an unreasonable price to pay. :-(
Many of the people in the Middle East don't truely grasp the size of the United States - many of their countries are relatively small, and even if they do have a significant land area much of it is uninhabitable. I suspect that many of them - even those who have spent a little bit of time here - don't really grasp the size of such an undertaking. There are various family stories about visitors from that part of the world asking about taking an afternoon trip up to Niagara Falls from oh, say, Pittsburgh and being astounded that it was a 450 mile round trip - they look so close together on the map. :-)