Their conclusion: "These findings suggest that ADHD is best viewed as the extreme of a behavior that varies genetically throughout the entire population rather than as a disorder with discrete determinants."
As someone who has been diagnosed with ADHD, this seems like a more natural explanation. If 5-10% of the population allegedly have a disorder, then maybe the "disorder" is in fact just the tail end of a normal genetic distribution. Doesn't mean that the condition is "made up" or that those people don't deserve some kind of treatment, of course.