It's not confounding at all. It's making the point that internal motivation, according to the study, has no major factor in savings regret.

It says that understanding risk (as operationalized by understanding probability) has a larger effect.

But it is also saying that the more external impact someone has, the more they regret saving more -- in the United States but not Singapore.

The study is explicitly saying that internal motivation does not seem to matter. And the article is arguing the reason why.