that poster started off with "It's not a coincidence that you don't see Christian suicide bombers, believers anxious to martyr themselves and killing cartoonists."
The response was that in America, Christian terrorism is much more common than Islamic terrorism. And instead of recognizing that weakness in his argument, he came back with 9/11. I assume he means that the total death toll from Islamic terrorism is higher (though that is almost certainly not correct any more). Pointing out that the death toll from ordinary gun violence (which he does not meaningfully oppose) is even greater demonstrates that the body count approach is not the right way to look at it.
Let's suppose for a minute that the 9/11 pilots weren't so good, and they hit the towers up at the very top. The towers would eventually fall but most everyone would have been able to evacuate and the death toll was only 75. Does that make the act any less terroristic? Would it be less of a big deal just because they missed? Or what if they hit on a national holiday that they didn't know about and the towers were mostly empty? Again, how exactly does that change the nature of the act.
I'd say this consideration pretty much blows the "but 9/11" nonsense out of the water.