What is being said is not simply that people who engaged in a certain activity since childhood do not become top performing adults. Obviously that happens a lot. But rather that the top child or youth performers are not reliably the ones that turn into top adult performers.

Let me express it another way.

Think of 5 relevant attributes of your body for playing something well.

Guesstimate where they were on the population bell curve when you were 10.

Guesstimate if these would have been on a different spot on the population bell curve for that attribute when you were an adult. Would you have guessed it when you wee 10? Would others have guessed it about you at that age?

Puberty changes you in unpredictable ways. Do we need a study to know that?

Everyone committing to tennis before they are 10 are elite, you wouldn't do it otherwise. Who is the best player of that elite set changes given the great puberty shake up.