This study isn't surprising, I'm unsure what you think it implies. Indeed IQ researchers have been wary themselves and known about motivational effects for decades, along with many other objections to testing like cultural bias and the like that in good modern research are all accounted for -- obviously if I sleep during a test I'll score very low, but if I'm actually awake and care I can increase my score by a huge factor. IQ isn't a perfect correlate, but it's common to reason as if it alone has a predictive power of 0.4-0.6 for various important things, that is stronger than any other single factor we know about. When you add in Conscientiousness (the big five traits being the other contender for psychology's crown jewel, I think), which is about grit, intrinsic motivation, and the like, together with IQ (two factors now, not just one), you get predictive powers of 0.7 to educational success. I would wager that the differences seen in the paper cited by your article are almost entirely accounted for by Conscientiousness, but from the abstract (don't have the full paper) it looks like that was not controlled for at all.

For your pleasure: https://ideas.repec.org/p/nbr/nberwo/15898.html