If the success has a lot more ADHD-tendency than the failure, how do we defend the idea that the failure has ADHD, and the success doesn't?
It's just shorthand for "this is something we'd like to fix".
The criteria is more like “but for this tendency, the patient would be/feel better, all else being equal.” A software engineer making $$$ could meet the criteria if their inability to focus keeps costing them promotions, launch their own company, or whatever their goal might be. A neurotypical buggy whip maker who can’t find or hold a job doesn’t qualify, even if he is obviously worse off overall.
You have two people with ADHD-like tendencies. One person feels they have found their own ways to adapt to their ADHD-like tendencies and that they are successful. Another person feels that they have not found ways to adapt to their ADHD-like tendencies and their life could be better if they found ways to adapt.
In this case, the actual magnitude is less important than how individuals adapt.