IIRC (and if I wasn't so darn lazy I'd find the article and re-read it) he somehow reached that conclusion because terrorists can come into the country at their ease. But elsewhere he said that we are safer now because it's harder for them to enter...??
Apparently he thinks there isn't any more real threat as long as they don't come in the country from now on. Then why is he spending so much time and effort studying up on them?
And what about the ones being converted as we speak? He did say that most of the ones he studied were converted as adults, often in foreign countries.