So, no, he's not automatically dismissing any counterargument. He's saying that the ability to rationalize anything is a strong reason for immoral behavior.
If I killed somebody and tried to rationalize it (he would have died at some point anyway, maybe he was a bad person, etc.), you could retort that no matter what, I killed someone and it's just bad because you are causing lots of harm (the victim, his family and friends, etc.), but you couldn't tell someone who downloaded illegally some music that it's just a bad thing to do.
That's why I agree with the GP that his mention of the rationalization process is here to automatically dismiss counter argument. It is an interesting point, and I am sure people used this process to have a clear conscience having done really bad things, but it is not by itself a counter-argument to illegal download, or anything else.