Not necessarily. Can you not run because you have mild asthma / a weak heart, or because you never ran? It's hard to tell the difference between the two. Same here.

Over-diagnosing is a real problem, it's the opposite end of "victim blaming", but it's also a dangerous extreme.

There are people who can't focus on anything who don't have ADHD. Willpower is directly trainable through a number of things - working memory exercises, physical exercise, practicing it, meditation, etc.