"I am also aware that many leftists firmly believe that the Islamist threat has been entirely generated by America's imperialist foreign policies, our support of Israel, and the evil Bush, and that if only some titanic intellect like Kerry or Hillary is put in charge, it will all just go away."

Name one.
(If you can find anyone this side of a mental asylum who thinks that it will all go away if Kerry was in charge, I'll eat a box of hats).

How the heck do you get from that to this: "He doesn't think anybody believes that the Islamic threat is the fault of U.S. policies."?

I could probably name lots with a little research, people who think that Islamic threat is the fault of american policies.
There would be more still who think that US policy over the years - particularly wrt Israel and Iran - has helped to create the idea of The Great Satan and aided in the radicalization of Islamic groups.
I could find many many more who think the invasion of Iraq was a supplemental and contributory factor to current Islamic opinions of USA.

But since you increasingly only believe in a black and white, with us or agin us fantasy world, there's not much I can say to persuade you otherwise.