1. Disorders can be disabilities.
2. Medication and disabilities are not opposites of each other.
3. Disabilities are a spectrum, yes. If you have ADHD and don't personally want to consider yourself disabled, then fine. But don't go telling everyone that ADHD isn't a disability at all.

ADHD is literally listed as an example on the CDC page about disabilities. https://www.cdc.gov/disability-and-health/about/index.html