I'm sure he must know the poem. He was a Middle-East scholar at Oxford, and wrote some of his first books about that area.
Here's the link, from Hitchens' column (in The Nation) of Oct 4, 2001:
There are others who mourn September 11 because it was on that day in 1683 that the hitherto unstoppable armies of Islam were defeated by a Polish general outside the gates of Vienna. The date marks the closest that proselytizing Islam ever came to making itself a superpower by military conquest. From then on, the Muslim civilization, which once had so much to teach the Christian West, went into a protracted eclipse. I cannot of course be certain, but I think it is highly probable that this is the date that certain antimodernist forces want us to remember as painfully as they do. And if I am right, then it's not even facile or superficial to connect the recent aggression against American civil society with any current "human rights issue."
http://64.233.167.104/search?q=cache:ipg69JBY7TAJ:www.thenation.com/doc.mhtml%253Fi%3D20011022%26s%3Dhitchens+christopher+hitchens+september+11+1683&hl=en