The youngest person in the study was born in 1952. All ADHD and dementia diagnoses were after 2003. The relationship between dementia and ADHD wasn't mild. So

The youngest person in the study was born in 1952. All ADHD and dementia diagnoses were after 2003. The relationship between dementia and ADHD wasn't mild. So the claim a mild relationship would have been impossible to detect is irrelevant even if true. And they didn't say to to take the less elevated risk in the stimulant taking ADHD group as conclusive evidence of a real effect. Dopaminergic stimulants and modafinil are nootropics prescribed to treat dementia also. And you would need to suppose those drugs were prescribed more to people without ADHD or dementia diagnoses to believe this explained the elevated risk of dementia in people with ADHD.