He's not entirely wrong. If we had a good way of getting a warhead to an incoming asteroid, it'd be a great safety mechanism. Unfortunately, today, there is no one trustworthy to put that sort of weaponry into somewhere that would allow it to strike any location on earth with less than a few minutes warning. Especially since something of that yield would devastate a large portion of earth.
Though lets be honest, anything in orbit is dangerous. A crowbar (aka Rods from God aka https://en.wikipedia.org/wiki/Kinetic_bombardment ) is even smaller and just as dangerous.