Not fitting into Capitalism has nothing to do with common ADHD traits such as lack of emotional regulation, time blindness, and our tendency towards risky behaviors.

All of those traits generally make you a less predictable and harder to manage worker. Capitalism absolutely cares about those traits very significantly.

I could be anything from an Mesopotamian King to a rider in the Mongol horde to a Medieval Peasant and my brain would still have the same damn problems.

This is literally impossible to say. Your brain would still work similarly, but it would have been shaped by entirely different stimuli and environment, so it would not be working the same. It's also entirely possible that those tendencies are beneficial in different historical contexts. Other systems like feudalism may also similarly view ADHD as a detriment, but that doesn't mean that every system / context does.