This is no way disregards the difficulty of living with an actually severe case of ADHD but is not what most of these people are dealing with.
EDIT: many seem to have misunderstood what I mean by this. I'm not saying these people are only claiming to have ADHD to use it as an excuse. What I mean, is that they may very well do have, and they're using it as an excuse. Mostly to themselves.
Making this about me doesn't refute my claim either.
I don't believe in free will, thus nothing is under my control. I still try and get stuff done according to my best abilities despite my struggles. Telling a fat person that it's about their genes and there's nothing they can do about it is not true nor helpful.