The title is somewhat misleading -- it's not like Kindergarten causes ADHD. The key observation is that if you grab any group of young kids with an age spread of a full year, "younger children may be mistakenly diagnosed as having ADHD, when in fact they are simply less mature." You cannot just pretend they're all at the same stage of development.
You see a similar effect in sports (where again, older kids in the group will have a big advantage).