You are correct that these traits would apply to any arbitrary group of young children with a one year age spread. Kindergarten is an ideal such group, not only because it encompasses a one year spread of young children, but also because it is a group that is being observed for signs of ADHD.

But yeah, I'm not trying to say that the title isn't misleading - there is a correlation between Kindergarten enrollment dates and ADHD diagnosis and that's it. The article makes this pretty clear but the title does not.