The first part of their finding, that motivated people or those with an incentive will do better on IQ tests, does seem common sense.

The second part of their finding is more interesting to me, although I'm still trying to fully understand it. I think they are suggesting that innate heritable intelligence isn't as predictive of life outcomes as previously thought because part of what was being measured was motivation to do well on IQ tests.

In other words, two people could be born with the same innate intelligence, but one of the two could be more motivated and do better in both IQ tests and life outcomes. Previously people would've attributed the difference in life outcomes to a difference in innate intelligence. It still seems like IQ would be an interesting number to look at, but it isn't quite measuring what people believed it was measuring.