The way he defines success and failure is framed to say all missile defense fails.
Iron Dome uses a combination of a proximity (radar activited) fuse and fragmentation. Sometimes the interceptor destroys the warhead. Sometimes it causes an explosion of the propellant which destroys the warhead. Sometimes it simply breaks the incoming missile or rocket into segments or destroys its ability to follow its planned ballistic path. According to Lloyd and Postol, if the warhead isn’t destroyed the interceptor failed.
You don’t need a Ph.D. to see the immense flaw in this logic: if someone fires a missile at you and you aren’t hit that is good news.