I'm sure there's partisanship and tribalism among atheists, as there certainly is among theists such as Christians. But there are also genuine adherents to those opposing viewpoints. We're all likely to defend our convictions; otherwise, we'd have no basis for acting in the world, no understanding of what's happening. But some folks are more objective than others, more understanding of how critical thinking and standards of evidence work. I suspect atheists are, on average, a lot more self-critical than Christians. In any case, this psychological question is something of a red herring in the context of this article, isn't it?