Logically, it is a valid statement, just as much as "Christians killed civilians in Iraq" or, more pointedly "Someone wearing jeans and black socks is posting in this thread".
The point it: the fact stated has little or no relevance to the action committed. Yes, the terrorists were radical muslims and part of what is called Al Qaida. However, terrorism does not require Islam to work. There are nationalist terrorists, christian terrorists, hell, there even have been radical pacifists who qualify.
The way Mr. O'Reilly used the sentence is not in a logical sense, though, but in a political, spin-doctored sense. He omits the context and just throws a technically true, but essentially unconnected statement out there. Or, to return to the imaginary quotes above, essentially claims that civilians die in Iraq because some American soldiers are Christian or these forums only get posted in by people who wear jeans and black socks. Just one of many ways to bend an argument.