Even if that's true, it still means that there is at best a tiny advantage to an asteroid weapon compared to a nuclear weapon - and a gigantic disadvantage in terms of energy cost of moving an asteroid from its orbit to Earth's surface, and in terms of the months or years it would take to deploy (in which time your enemy might even find a way to stop you, or at least launch all of its nuclear arsenal on you if that's not possible, MAD style).