This claim is mainly the result of when the school year cutoff is.  Much like how an unproportionate number of successful athletes are born right before youth sports leagues have their age cutoff.  This is because it allows them to be slightly older than their peers and receive special attention because of their perceived elevated skill level, when in reality they are just older. 

However, you can also find articles that directly dispute your claim, such as this one: https://www.marieclaire.co.uk/life/sex-and-relationships/babies-born-most-successful-months-699851