Your interpretation is too weak. It is not just "not necessarily better", it is the stronger "on average are probably worse".
The reason is that programming competitions give more of a boost to your odds of being selected than to how well you'll do on the job. So people who otherwise wouldn't have gotten in now will, and will not perform as well as the people that they displaced.
Which is what happened with hockey players. Being born at the right time of year put you in a bucket with people who were slightly younger than you. Which improved your performance on the tests, but didn't matter once you all grew up. So slightly worse people at the right time of year displaced slightly better at the wrong time of year, and the average came out that people who got through and were born in the latter half of the year were actually better.
The hypothesis is that the older kids are bigger stronger but the younger kids are more skilled. And apparently this holds up all the way to the NHL where the skill edge overcomes age.
The reason is that programming competitions give more of a boost to your odds of being selected than to how well you'll do on the job. So people who otherwise wouldn't have gotten in now will, and will not perform as well as the people that they displaced.
Which is what happened with hockey players. Being born at the right time of year put you in a bucket with people who were slightly younger than you. Which improved your performance on the tests, but didn't matter once you all grew up. So slightly worse people at the right time of year displaced slightly better at the wrong time of year, and the average came out that people who got through and were born in the latter half of the year were actually better.