Nic_Smith beat you to it ("it" in this case being a Discussion post and link, not your particular criticism of the argument).
An article in today's New York Times summarizes the "argumentative theory of reasoning", which says that "biases" in reasoning are adaptive because they help persuade other people to adopt your opinion.
I don't think this makes sense. If we evolved biases as argumentative tricks to win others over despite not having the facts on our side, we should also have evolved the ability to see through those tricks.