What's the applicable result to this? Put children who eat marshmellows too quickly in special "delayed gratification" classes? Is the result such that all children who eat marshmellows too quickly should give up, as their success is predetermined, or is it the impact is barely statistically significant?

If I were the parent of a child who ate the marshmellow too quickly, I would do nothing different, as the whole thing is meaningless and doesn't change anything. It's more like, "Oh, OK, my son is 4% less likely to be in the top 20% of the income distribution (according to this barely coherent theory)." Why do we spend so much money, time, and effort discussing and researching this?