The joke here is that the bond traders could not foresee 9/11 and did not predict the response to 9/11. They were forecasting the recession based on predictable indicators.

And 9/11 itself couldn't possibly have caused a recession. 2 buildings just isn't enough damage to be measured in a system as large and complex as the US economy. If you want to argue that the Afghanistan invasion contributed to the recession I am sympathetic to that idea, but the general consensus seems to be that war is helpful for the economy (which I don't hesitate to argue is a stupid consensus - but it is what it is).