A person with ADHD physically cannot just sit still. They desire to stay focused and when told to do so, they try as hard as they can to obey but their mind will still wander and their body will need to move. However, with the use of medication in the most severe, they now suddenly can focus, sit still and do exactly what they desire to do (obey). It actually assists them by overcoming the physical issues that are happening in their body, thus allowing the brain to work properly.
That is a sin?
A person with ADHD (or that person's parents) are looking for a reason why they can't maintain control of themselves. They lack motivation and self control. Yes, it is a sin.