It is one thing to say that there is height variation, which is clear to anyone and easily empirically tested. However, saying that the quality of someone's height is attributable to genetics, diet, diet of the mother during pregnancy, social standing etc, is a bit harder to ascertain. The same with this research. Saying there are smart people or stupid people is not what the article concludes. They are already testing within a selected nonrandom group (college kids).
The claim they are making is that prior knowledge is a very big differentiator in "speed of learning".
Now, to claim this means that the kids with more prior knowledge are not "smarter" is a very weird claim. Smartness is not some metric that exists without a society. We all know the stories of the feral kids. Smartness is mostly cultivated, this article underpins this further by identifying prior knowledge as an important factor. So important, that other differences seem trivial. How the kids gained the prior knowledge is not part of this study and could be the focus of a future study.
“Some people are taller than others” is dogma?