The Jarvis article cites government statistics stating that Arkansas and Kentucky are the states with the highest rates of ADHD diagnosis. In these states a high percentage of kids age 6-15 (over 85%) fish and hunt (over 30%). He compares these numbers to those in New Jersey and Nevada which have the lowest ADHD rates in the US. In these states only 44% of kids these ages fish (it does not state how many children hunt there). Jarvis holds this up as proof that ADHD does not exist as fishing and hunting require a high degree of focus to be successful.
This blog uses correlational data at the state level all the time to show relationships between variables that show meaningful patterns. I am very careful not to state that it conclusively proves or disproves anything the existence of a disease. Even further I would not limit my analysis to four states to make such a claim.
There is centuries worth of research backing up the existence of ADHD. The diagnosis of it is still not an exact science with it being based on behavioral observation and neuropsychological test data. Genetic abnormalities have been found which are associated with the presence of the disease. Someday genetic testing and brain scans will be able to provide a more definitive diagnosis but we're not there yet. ADHD children and adults can exhibit a trait called overfocusing and not all are hyperactive.