This is certainly interesting, but I think that we're jumping the gun by saying that "observation by parents is much more reliable than observation by trained professionals" based on this. For one thing, only 60 of the 219 subjects were observed by healthcare professionals at all, which may account for some of the difference in correlation.
Also, out of the three possible explanations proposed, you've chosen one: "parents may be 'particularly attuned to the idiosyncrasies of their own children'", seemingly because it is the one that matches your personal experience best. This is a textbook example of confirmation bias. We should not conclude that parents are experts on observing their children's hyperactivity without investigating the possible explanations.
Let's examine one of the other hypotheses:
parents may detect behaviors that are not listed in the DSM for ADHD.
I'd like to make it clear that this does not necessarily mean that parents are better at detecting changes in their children's behaviour than professionals. It could be that they observe changes in behaviour that are unrelated to ADHD or hyperactivity, and that those changes lead them to the conclusion that their children are not part of the control, at which point the placebo effect becomes relevant. Another possibility is clearly stated in the article:
AFCs may promote a pattern of symptoms that is incongruent with modern criteria for ADHD but that is nevertheless bothersome to parents.
This may suggest that the modern criteria for ADHD are insufficient, but it doesn't tell us anything about parents' knowledge or observational skills in general. If the difference can be accounted for by additional criteria used by parents, then professionals may get similar results if they use the same criteria. In summary, if the trouble with professional observation is that ADHD is poorly or incompletely defined, then the discrepancy will change or disappear when we examine behaviours that are unrelated to that particular disorder.
Finally, this is a meta-study of one particular sort of behaviour. To conclude that parents are generally better at observing their children's behaviour than experts, we would need to examine a wide range of behaviours. We can form a hypothesis based on this study, but much more investigation is necessary if we are to come to any conclusions.
It was "Some folks believe in nothing / But if you believe in nothing /Then what’s to keep the nothing from coming for you", which seems pretty obviously sarcastic to me.
The fact that it's 15 small studies rather than one large one actually works against it. Since the studies were conducted differently, the control is shaky.