Nah. There are plenty of intelligent students who don't have ADHD but are either lazy or rebellious enough to not care about conventional measures of academic success.

Sure isn't.

I annihilated the SATs. My grades were only good in high school because I was just "gifted" enough to get As without studying. I do not have and never had ADHD. I also never learned how to study.

I almost failed out of college. I didn't know how to study. I didn't have the habits. I sure had a lot of fun in high school and college though.