No, it looks more relevant to the fact that everyone starts school at the same time. Clearly, it would appear that the extra months of development for the elder children has a knock on effect throughout their educational careers. It would be interesting to see if a similar impact was felt in GCSE results/SATS/A levels etc.
If I’d have been born a few days later I would have had to wait another year to start school, so I was easily the youngest in my class. Yet I had the highest ACT scores my school and ever produced (at least up til that time- I don’t know about since).
Still, that might be the “exception that proves the rule.” I agree that the extra developement time gives one an edge. I just happened to be precocious.