It depends if you want to be ackshualy technically correct or conversation correct. We won't confirm causality until we flip people's genes to give them ADHD or describe the whole exact interaction at whole-organism level, so practically... never.
On the other hand, for normal purposes, we know it's inheritable. You'd need to discover some whole new mechanism for how diseases could possibly transmit to prove otherwise. There's enough real world data to say this.
Conversational or otherwise, you keep saying it's inheritable but these "ADHD genes" are only correlated with increased risk, not predictability. That's key, because one is nature, the other nurture.
Also, proving a correlation doesn't necessarily mean it's exclusively, automatically the only variable. That's the tricky bit with confounding variables: they could be one of several, or many, correlative effects, and where none could be the cause -- much less a useful avenue of solution. It's helpful, but not conclusive.
Great, we found some genes. But other studies should also look at psychotherapy, or even diaphragmatic breathing (asthma is another great example). Alternative analysis, i.e. anything other than the most lucrative option for pharmas to sell drugs -- as in, alternative solutions that are closer to the root cause, not just the most complicated.
On the other hand, for normal purposes, we know it's inheritable. You'd need to discover some whole new mechanism for how diseases could possibly transmit to prove otherwise. There's enough real world data to say this.