What is the skill they should teach ? To always eat lunch before doing a psych experiment? To like sugar, or marshmallows in particular, just slightly less? That high self-control in extremely low-reward situations is good?

Teaching a child to do basic budgeting with pocket money is likely to have a vastly higher impact than slight delayed gratification with candy.

See hgsgm's comment. Just because the test predicts outcomes doesn't mean what it's testing is causing the outcomes.

But that's precisely what is so ridiculous. Eating marshmallows slowly is not a valuable life skill.