I would argue that it is mostly about amount of ball involvement while playing. A kid born earlier in the year will on average be physically stronger and therefore have the ball more and therefore develop skills faster, even if players born later in the year plays an equal amount. When the player born later catches up physical the early born players will have better football specific skills which is hard to catch up on.

Something else the book argues is that opportunities were very important, and you can also think that it was the opportunities given to those who had more physical development at the earliest stage increased their likelihood to make it.

Absolutely, I even reference this.

This is sort of like her getting selected for the special team outside the odds.

Yep. She's the Outlier - geddit!?