Well, both gwern's essay and the essay linked by OP assume that both sides have ships capable of interstellar travel. In gwern's case the idea is to not bother with the ship; just bolt an interstellar drive to a comet and point it at the enemy's homeworld.
As for them throwing comets back at you, that is addressed in the essay. In short, there is nothing stopping them from throwing comets back. Space war will resemble the Cold War much more than any active engagement. Mutually assured destruction will be the overriding doctrine.
As for them throwing comets back at you, that is addressed in the essay. In short, there is nothing stopping them from throwing comets back. Space war will resemble the Cold War much more than any active engagement. Mutually assured destruction will be the overriding doctrine.