No, it doesn't. You show someone he's out of his depth by attacking his argument, not by pointing out that the counter-argument comes from someone famous. Are you seriously claiming that no one you've never heard of can possibly argue against a point made by someone you've heard of? That's a sheepish mindset.
The fact that Rob Pike wrote a usable language that is not OOP in the sense that the commenter thinks of OOP, in which you have composition without inheritance is direct evidence against the commenter's point of view that to do composition you need to be doing OOP.
But appeals to authority are very shallow arguments. He should still be able to out debate someone who doesn't know who he is.