Another way of looking at this is that even if it were somehow true that the majority of people born in September were of higher than average intellect this would not mean that they are correct about particular issues. Even smart people make incorrect conclusions; and it has been shown both in the history of philosophy, and in experimental psychology that no one is a purely rational being, untainted by emotions, desires, and the will. That just isn’t how our reason works, and we are far too complex of creatures to think we come to an issue as purely neutral and rational minds. No one believes this to be true. Also, if Christianity is true (which I think it is), then you also have the noetic effects of sin to deal with.
Furthermore, what about all the Christian scholars who have been born in September? I don’t know when most NT Scholars were born, but let’s assume we find out that someone like N.T. Wright or Craig Bloomberg, or Mike Licona was born in September. Now what? How do we adjudicate between the conclusions of September babies like Ehrman, and some other September baby like a Wright or a Licona?
Finally, I think we can probably say with great confidence that the idea that people born in September are smart because they are born in September is a classical example of correlation, but not causation. In other words, it is just happenstance. Also, I doubt that the sample size is large enough to show that this is true; it sounds like an incredibly Hasty Generalization to me.