This is wrong, studies have shown that ADHD brains are fundamentally different from non-ADHD brains in the sense that they have reduced frontal lobe (read: executive functioning, ie. cognitive inhibition, working memory, decision making) matter.
There are a core set of people who definitely have brains that are different and need medication etc.But the way it is currently diagnosed lumps too many people and makes it useless to call all people with ADHD the same.
My son with Generalized Anxiety Disorder was diagnosed with severe ADHD. This is the problem because many things can cause the same set of behaviors so a diagnosis of ADHD is not accurate enough.