It's quite an important distinction tbf. Yes it affects your mental health but it's not a mental health disorder like depression or anxiety, it is a developmental disorder which is caused by physical dysfunction in the brain.

I think to say that they're telling you that ADHD isn't real is a bit of a stretch. I've found it immensely helpful to realise that the symptoms that I experience aren't just laziness or dizziness and have a physical cause, and that it's not something that I can go to see a doctor and be given medicine to "recover" from (as well as you can recover from depression or anxiety or other common mental health problems people experience).

Classing ADHD as a mental health issue just lumps it in with things like depression and anxiety and imo could give the impression that I should work harder at seeking treatment because doing so will cure me, as so many seem to mistakenly claim about mental health disorders. It won't. I can manage my symptoms but this is how my brain functions and this is how I will always be.