That wasn't the question.
The question wasn't "Why is it so over-diagnosed?" it was "Is it ACTUALLY over-diagnosed or is that just what people SAY it is?"
I'm fairly confident that ADHD doesn't effect 10% of the population (that's roughly how many American children the CDC says has been diagnosed with ADHD), although part of my argument revolves around the definition of a disorder.
To me, for it to count as a disorder, it would have to be intrinsic, AKA "your mind works this way" as opposed to "you act this way due to an external situation". The thing is, ADHD appears to be tied to location. Within the US, it becomes more common towards the east coast:
Prevalence of ADHD also appears to be tied to time, aka less adults appear to have ADHD than kids. And it seems to be a modern thing. If you look at mental states in the past that were decently common but weren't understood, such clinical depression, addiction, or paranoia, they still showed up in stories of the time, just not named as disorders. And those conditions (with the possible exception of addiction) affect way less than 10% of humans. ADHD doesn't seem, to me, to have the presence in fiction across the ages that it should if it affected 10% of the population. Although, granted, ADHD can be dismissed as laziness or general distractedness, but way less than 10% of characters in fiction are portrayed as distractable/lazy people.
So while I would agree ADHD is real, evidence suggests to me there is an external situation that produces ADHD like symptoms in those that do not actually have ADHD. Given that this situation appears to occur in a specific part of the US and mostly applies to school aged children, my conclusion is that kids who don't fit into the school system are incorrectly being diagnosed with ADHD.