It’s not that free radicals are good (they are necessary, but excess free radicals are definitely bad), but more so that there is no solid research to suggest that dietary antioxidants have any effect whatsoever. All the studies that show any beneficial effect have been shown to have major flaws or have not been able to be reproduced consistently.
Those are a big part of it, it’s also that our brain chemistry interacts with stimulants differently.
One of the major effects of stimulants is the increase in availability of dopamine. The under-stimulated feeling comes from a lack of dopamine due to the reward system not functioning properly in ADHD. When the stimulant forces a release of dopamine it’s not as “stimulating” to an ADHD brain that didn’t have much dopamine to begin with, whereas a neurotypical person experiences a huge abundance of dopamine and feels incredibly stimulated and energized.