Many cases of ADHD accompany sleep-breathing disorders, like apnea or UARS.

If not those, there's delayed circadian rhythms, insomnia, and just plain struggling to maintain a healthy lifestyle in regards to sleep.

We know poorer sleep leads to Alzheimer's. Drugs involving neurotransmitters like dopamine seem to play a role as well.

Then you've just got poorer general health. ADHD people are more likely to struggle with nutrition. They're more likely to abuse substances, alongside an increased risk of regular alcohol, nicotine, and excess caffeine use.

This study definitely didn't isolate a high quantity of people with ADHD that also had none of the above.

I'm not saying there is no link outside of these other things. But this is kind of to be expected. But it would be stranger if there wasn't, really