ADHD is a disorder that makes it difficult for a person to pay attention and control impulsive behaviors. Setting an intention and maintaining it for a prolonged period of time requires one to pay attention and control impulsive behaviors. Those with ADHD can improve in these areas to the point where this is feasible, but by doing this, they probably would have already cured their ADHD.

Think of it like exercise. One exercises to get stronger in areas where they are weak.