He is very wrong. He is confusing foot soldiers with their leaders. Terrorist leaders are not dumb.
Yes, it's harder than we think but not for the reasons in the article. In Israel, there was a wave of bombings every few days. The bombers were not dumb. They were very successful and killed many people. I believe that it wasn't until the borders got sealed tight that the bombings subsided. That's why they had to resort to using Katyusha rockets but, believe me, if they could get in the country, they'd blow things up, no problem.
Iraq, Afghanistan and Pakistan all have had regular waves of successful suicide bombings.
What keeps the US relatively free of terrorism is the massive ocean that's between the terrorists and their target. It makes an operation much harder to execute because it's more remote from their base of operations. What we've seen so far is the evolution of their attempts to bridge that gap. The author is confusing these early experiments with stupidity. The terrorists are smart and they learn from the failures of their experiments. Eventually, they'll get better at blowing things up from a distance. If we write them off as dumb, we're going to be in for a surprise when they perfect their methods.
Edit: Israel is tiny and could control its borders much easier than Iraq, Afganistan, Pakistan and the US. We see that Iraq, Afganistan, Pakistan are very vulnerable to terrorism because their borders are easy to penetrate. The US has the advantage of an ocean defending its borders. Eventually, they'll overcome that obstacle.