Diagnosis means access to more SEND help and support, it means understanding that you need to parent differently, it means that a child who is struggling to learn and fit in knows it's not their fault.
Meds help said person to focus, to be able to learn, to remember, to have the same opportunities as other children, to not self medicate or seek the dopamine hit that their body does produce. Depending on the type of meds they can reduce tics, and calm disregulation.
If someone who "works in mental health" doesn't know what ADHD is or how the meds work or the science behind them then it's a huge worry.