Pingviner, quite right, she's apparently the grand old age of 21, she's done a psychology course (not a degree yet, obviously), and she's not read the literature on the subject properly, as demonstrated by her failure to cite any mainstream work at all, such as the chapter on Rutter. Just encountering 300 kids in school or pre-school does not constitute evidence.
She may indeed have thoughts and/or opinions on ADHD (Haven't we all? Isn't that sort of thing what MN is for?) but she did not articulate these very thoughtfully at all, which further undermined her position, and made her comments appear quite judgemental and unkind. The big question is why is everyone engaging with her at length like this? She hasn't even come back to the thread.
As an aside, one thing nobody has flagged up so far is the concept of difficult temperament, which is not technically a disorder or condition, but a (possibly) genetically influenced factor in a child's behaviour that presents similarly to ADD or ADHD. There has been some research done on this, but not a great deal so far (maybe because it's overshadowed by the ADHD/Ritalin industry). However it may well be that the 'case study' the OP describes involved a child with this, rather than any kind of ADHD. That doesn't mean going after a label or diagnosis is wrong, because the strategies for helping the child are largely the same, and labels are useful as a shorthand for indicating the kind of help required.
So it's indeed a lot more complex than the OP realises. But I think many of us are aware of this.