As The 12 Virtues of Rationality says, “Because this world contains many whose grasp of rationality is abysmal, beginning students of rationality win arguments and acquire an exaggerated view of their own abilities.”
Since they’re right, atheists are more likely to win arguments with theists. They may then think that they’re smarter instead of that they’re just on the right side of an issue. They may even think they know more about other things than they really do.
And if they spend a lot of time mocking the “enemy side” with insults about how stupid, insane, and evil they are (what, all at the same time?), then they may have an even more “exaggerated view of their own abilities.”
The more right you are about an issue, the more you have to watch out for overestimating yourself on other issues. Since atheists have the right answer to the God Question, they have to be more careful about thinking they’re right about other issues.