Exactly. They controlled for socio-economic inputs as if that was an independent variable. But the entire point of the original experiment was that delayed gratification led to better socio-economic outcomes.
"Ultimately, the new study finds limited support for the idea that being able to delay gratification leads to better outcomes. Instead, it suggests that the capacity to hold out for a second marshmallow is shaped in large part by a child’s social and economic background—and, in turn, that that background, not the ability to delay gratification, is what’s behind kids’ long-term success."
We already know that socio-economic outcomes are tied to parents. Successful parents tend to produce successful children. It's not independent at all. So in "controlling" for that input they likely just controlled out the original finding, even if the original study was correct.
This seems to be a big logical flaw in this study design.
Agreed. This is a BS study parameters and conclusion.