Marshmallow willpower is still correlated with future success, but only because it is correlated with wealth. This means that there's isn't a causal link between eating marshmallow willpower and success. which is a different result, as the previous result included that as a possibility.
If success is correlated with wealth, then wouldn't the original study (done with kids at a Stanford preschool) have shown a small/null result? That is, most of the kids have wealthy and well-educated parents.
