He "thinks that" because for him, and many many like him, a system that successfully neuters Hamas is very troubling; Iron Dome removes a pain point for Israel and a bargaining chip for Hamas, and Ted Postol and his NPR ilk don't like it. The fact that Israel has created the antidote for Hamas and their ballistic pipe bombs means Israel can exist relatively untroubled, and this reality is so disturbing to these people that they will grasp for and cling to any claim to the contrary.
In the long run they need not worry. This is an arms race and eventually Hamas will be given something better to use. The qassam rockets themselves were just another step in that progression after Israel isolated itself from Gaza suicide bombers with the West Bank wall, another successful and effective Israeli creation that NPR routinely lambastes.
Eventually Hamas will be given new weapons, Israelis will start dying again and all will be well. Until then it's all hands on deck at NPR to downplay and discredit Iron Dome, and if you've got a claim that will convince any of the vast hoard of Israel Derangement Syndrome sufferers that Iron Dome doesn't work, they have the air time.
Israelis have interwebs just as good as you and, unlike the subjects that live in the surrounding dictatorships, theocratic monarchies and failed states, they're allowed to use it. Despite this there is very little video evidence of damage in urban areas and Israeli civilians aren't being killed. Given the quantity of rockets fired and past performance of qassam barrages, it is not possible that the rockets are getting through in large numbers.
"But they're covering up the deaths!!!!1" you say.
Ok, well, the burden is on you to prove your conspiracy based argument then; what's the "real" body count and were are you getting it?