Most of these people do fairly well in life. Their skills are extremely valuable and prized. They also almost always (95-99%) did not do very well in school. There are a couple of explanations:
1. Studies like these simply track educational achievements, which do not necessarily correlate with success in life.
2. I only ever see/meet the success stories, each of which there is a much larger amount of failures for.
3. There are multiple conditions that are all erroneously grouped under the moniker of ADHD. Most, if not all, of the successful ADHD people I know lean more towards the inattentive side of things.
4. Some combination of the above theories.