I think the reality is that everyone benefits from amphetamines whether they have ADHD or not, so working backwards, if you feel better on Adderall, that doesn’t mean you have ADHD.
That said, GP is talking out of their arse and is perpetuating one of the biggest reasons I waited until I was 35 to get diagnosed: I too thought ADHD was bullshit until I was in a position at looking at my life, my difficulties and how other people were living, and it was pretty obvious I was missing something.
This ignorance surrounding ADHD is widespread, and people think we take amphetamines to write code 80 hours a week without taking a break.
I take amphetamines to have the energy to work 4 hours a day and pick the trash off the floor while not hating myself, so comments like that one feel quite ignorant, yet you can find them in every HN thread offering their misinformed 2 cents.
In my case psychologist made the suggestion I had ADHD after 2 years of therapy, so they knew me pretty well by that point. It all made sense to me, and I went to a psychiatrist to get diagnosed and medicated.
Had I gone to a psychiatrist directly, they probably would have prescribed an antidepressant without spending 2 years getting to know me.