Looking at the ADHD example, aside from the genes, consider what else is consistent: same gestational environment (mother's food, air, water, emotions, microbiome), possibly the same living environment in early life (I'm not clear on how many of these twins were separated, and if so, at what age), but even if they were separated, they're still living in the same time in history, likely the same city/country, likely at a similar socioeconomic level, they have the same appearance/height (so people respond to them similarly) and if they were separated, both experiencing trauma of separation (both from a sibling, and one or both from parents).
So there's a whole lot going on that can explain the high heritability figure.
But even if you disregard all of that, you still have to explain the other (at least) 20-25%.
I was thinking about mentioning that because 75-80% is extremely high as far as I understand.