"You're just using it as an excuse" is now a red flag for me that I'm talking to a garbage person and saves a lot of emotional energy kicking them out of your life.
My response is even harsher, I go with lead has a 60% chance of causing adhd and lead was banned from being put in cars at the gas station in 1996. Then watch as most people my age realize they may have been exposed. The rest of the 40% chance is autism and more severely cerebellar hypodysplasia.
We actually do, though. Most mental illnesses--depression, ADHD, manias, phobias, anxiety disorders--are a matter of degree, not of kind. It's likely that some internal process isn't working at all for ADHD that does work most of the time in NT people, but it's still accurate to say that the symptoms affect everyone sometimes.
The rebuttal to this kind of argument is that you can treat the symptoms the same way in most people, and NT people often benefit from those as well. So even if everyone has ADHD symptoms, you can still treat them, instead of ignoring them, and it makes no sense to dismiss as unimportant problems that can be fixed.