In response to your edit: I think you're saying something very different than what your sibling responded to. The parent's claim was "They didn't correct for X"

In response to your edit: I think you're saying something very different than what your sibling responded to. The parent's claim was "They didn't correct for X"; your claim is "They tried to correct for X, but we have no way of knowing if they did it right." The difference is that what you're saying is true of any study. There are always obvious things to control for, and less-obvious ones, and the authors always try and sometimes they do it right and sometimes they don't and there's never any way to tell for sure without redoing the study[0]. The fact that a layman reading a mainstream article about the study can come up with a possible confounding factor doesn't imply much if anything about its possible veracity. We should be equally suspicious of all such claims, not only those which happen to have flaws which are apparent to us. [0] If the study contains a basic statistical error, redoing it might just be a matter of reinterpreting the original data; still counts.