Chronobiologist here. And also have ADHD.

The associations with ADHD really are there, but that may just also be because a huge number of brain processes have a circadian regulation, and those get disrupted by whatever is causing ADHD.

The causality is also bidirectional, because whatever ADHD does could cause light exposure behavioral changes, which could leave a circadian phenotype.

In general, a huge number of diseases have circadian phenotypes. To be a circadian disorder, you’d have to have clock mutations, and ADHD really doesn’t have those. So calling it a circadian disorder feels off.

But from personal experience, the linkage is strong enough to warrant deeper research, and beyond that, there’s some steps we can take ourselves. Regular walks in nature, going out to get sun… all generally useful things to do that do seem to help ADHD as well.