You have explanation right in that article

A few years ago, she and her colleagues found evidence that the marshmallow test doesn't involve just self-control. It also measures how much a child trusts his environment — how much the child trusts that the researcher is really going to return with the second marshmallow.

"What also matters is kids' expectations about whether waiting will be worth it or not," Kidd says.

Because if you think about it, if the child doesn't believe that second marshmallow is actually going to arrive, then it makes sense to eat the first one.

"We have evidence that kids take under consideration the statistical nature of what has happened in the past," Kidd says. "So for example, if a child is living in an environment where there's a lot of uncertainty and instability, then they may think that waiting isn't likely to pay off even though they have the ability to delay gratification."

Nso parents are quite strict and consistent with their discipline. So, Kidd says, perhaps this consistent parenting boosts the kids' trust in adults — that when the person in charge says they're going to bring a second treat, they likely will.

What I think is more likely is that this is a fluke result. Studies on self-control, whether Marshmallow test or real-life applications, consistently find racial difference in opposite direction.

Wany, Rieger, and Hens (2011) utilized a sample of 5,291 university students from 45 countries and gave participants a chance to choose an immediate monetary reward or a larger long term reward. This chart shows the proportion of people from different regions that chose the larger and less immediate reward. https://thealternativehypothesis.org/wp-content/uploads/2016/06/Wait-by-Country.jpg

Africa is at the bottom, Germany at the top.