This conclusion is only justifiable if (1) the population of "stone age tribes" and the part of the world where those selective pressures have been released (because people are indivisible units beyond a certain point), and (2) if intelligence was fully determined by genetics rather than merely having some heritable component.
If there are important environmental factors in intelligence to which modern technology can positively contributed, and there are a lot more people living with modern technology, then its quite possible that the mean, median, and max realized intelligence would be higher in the modern part of the world than in "stone age tribes", even if the latter have more genetic benefit from pro-intelligence selective pressure and thus a higher mean/median genetic intelligence capacity.
This follows from the two conditions:
1) intelligence is at least somewhat heritable (empirically true, according to the article)
and
2) intelligence is positively correlated with reproductive success (plausible give humans seem to have been selected for it)
The conclusion follows from the way world population has exploded in the past 250 years, increasing roughly ten-fold in twenty or twenty-five generations. That increase equates to "zero selective pressure": basically anybody could breed successfully, and they did (including my ancestors, some of whom occupied the more stagnant and brackish depths of the gene pool...)
So up until 1750 or so intelligence was being selected for, generation after generation. After that time, there was no pressure and the many small effects that this article documents as underlying the heritability of intelligence (with no single gene showing as much as a 1% effect on the overall outcome) would happily carry us all on a random walk across the landscape, creating engineers in some parts and political partisans in others.
This realization makes particularly funny the various organizations that have at times promoted Nobelists etc as pinnacles of intelligence (William Shockly, inventor of the transistor, was heavily involved in this kind of thing). Nobelists are certainly bright, but it seems more likely you'd find the most intelligent people amongst still-extant stone age tribes, whose pre-industrial life-style would still subject them to selective pressures long released in the more-developed world.