So, linkage studies indicate that ADHD has a multigene origin - many "hits" (mutations) are required to bring function to a low enough level that it is recognisable as a disabled state. (It's also highly heritable, supporting the claim that it's "real."). Similar studies have also indicated at least some of these genes are to do with dopamine regulation so the idea that this is all "made up" is a bit tendentious I think.
Because ADHD is a "many hits" condition, it's not very surprising that many parents of kids who have ADHD, also feel like they are a bit ADHD-ish. Parents may in fact have some ADHD traits, because they have some mutated genes. In a particular child, those genes combine to produce a loss of function sufficient to result in disability. Likewise, kids can sometimes compensate with other traits such as a high IQ. Having organised, highly functional parents (likely parents who do not have ADHD, albeit they may have some of the traits), may also be a resource that helps a child with ADHD to learn to cope with their condition. By the same token, if it's the parents who have few resources (low income, traumatic family background, low IQ, etc) then those parents will not be able to teach the child techniques to cope with their ADHD. So when a child comes from a family with "bad parenting," and then that child gets an ADHD diagnosis, that does not necessarily mean the child is a neurotypical child whose behavior has been messed up by "bad parenting" . It could be that the child has ADHD and the parents also have ADHD and/or other conditions that affect their ability to develop and teach coping techniques.
Another factor that should be apparent when you recognise that ADHD is a "many hits" condition, is that it's probably going to be on a spectrum - some people have really severe forms, other people have milder forms, yet other people have some ADHD traits but not enough to meet an official diagnosis. The diagnosis process uses the checklists to rate such behaviors on a scale and sum them to a "T score" but ultimately the question is whether the person's condition is disabling in their environment. What this means is that, as the environment becomes more and more stringent in the behaviors required to function, or the complexity of required behaviors rises, the threshold for someone's ADHD traits being disabling becomes lower, therefore more people will be diagnosed with ADHD.