Wouldn't this closely correspond to the Big Five "conscientiousness" trait? (Which the paper apparently doesn't mention at all?!) From what I've seen, even among the biggest fans of IQ, it is generally recognized that conscientiousness is at least similarly important as general intelligence in predicting success and performance.

That's an excellent point that completely did not occur to me. Turns out that self-discipline is actually one of the 6 subscales used to measure conscientiousness on the NEO-PI-R, so it's clearly related to conscientiousness. With that in mind, it is a bit weird that conscientiousness doesn't get a shoutout in the paper...