I tend to think so. On numerous occasions we have hosted a relatives' child for a few days to a few months. After some encouragement both firm and in love the child dropped all symptoms ADHD. Not always but in most instances.
Do you think some children being diagnosed with ADHD are really in need of lots of firm and strong discipline. I know there are hundreds of links and I've read hundreds. I was watching several TV programs and there was yelling, screaming and threats from parents and children, no discipline, no structure, no rules, just the child yelling and screaming for everything and then when he/she got it. It was over. Until the next time.
All children should be taught firm discipline, obviously it should be tempered to the individual child. ADHD is a disorder, NOT an excuse for willfull bad behavior, and the parents you described placating/rewarding the unruly child solved absolutely nothing, in fact, made the situation worse for the next time the brat wants their way. They have effectively taught the child the path to reward is to scream louder. This is not parenting, it is enabling. When parents are screaming, irrational, and then ultimately cave to the child's petulant demands, they have failed.
I now this will be an unpopular, outdated answer, but when I was a kid ADHD didn't exist. Teachers came to school armed with bamboo canes that they wielded with enthusiasm. We did what we were told. No drama. Surprisingly, perhaps, we even learned a few things.
My son had what now would be called ADHD, but we were fortunate, and the doctor said "What is causing this" rather than "what pills can he take" We had him tested. Artificial colours, artificial preservatives etc. Were what set him off. From then on, we watched what we bought (had to buy Heinz ketchup rather than cheaper ones as it had no colour added) It took a while to figure the different foods out (No internet to check, had to write letters with pen and paper), but once things were figured he was fine. (He was disciplined when he acted up as a child, as both my kids were)
Those TV programs tend to film the families that are way out in the tail end of the distribution and it's very hard to learn much about ADD or ADHD from those shows, much less how to treat either issue.
I was diagnosed along with our youngest boy when he was 7 and I was in my 40's. He was seen at the Shriner's hospital and we followed up with private physicians.
I had originally hoped that it was simply a minor discipline issue, but it was a real thing that he and I had. (And he was miles away behaviorally from the kids you see on those shows.)
After I had some awareness of what to look for, I attended a full day of kindergarten with him---ADHD is a VERY Real thing.
Some medication can be beneficial (neither of us takes it anymore). There are also various practical techniques to minimize the negatives that come with the typical difficulties.
Perhaps one of the nicest correlational benefits that come with the problem is an excellent memory and high intelligence---and that's much more useful than the problem is annoying.
ADHD can have a genetic element. I am in that family that has its fair share of ADHD people. We are not family of screaming, incorrigible brats. We are family that has been taught coping skills, and learned how to deal with the world around us. And now that there is medication that helps people focus. My dad was diagnosed in his 50s when my younger brother was. Ritalin has helped them both immensely.
Unfortunately some people will use the diagnosis as an excuse for bad behavior. They need to be taught how to deal with their frustration that the rest of the world does not function like them.