How do they know school are the issue if they didn't see any difference between kids who went to school and those who didn't? It's just their guess that's the factor.

Rather it's more likely as the brain matures they way you think changes to one which is more useful to you as an adult.

We already know the brain does this with observational skills. Children have much better observation than adults, they will notice pretty much everything which is going on around them. However this is highly distracting and stops them from being able to concentrate and focus so as we grow up we lose that ability for our own good.

Likely there's a similar trade off with imagination and creativity.