I have no PNAS*, but just for starters...

Firstly, this is such a massive and important finding, even cessation of the Flynn effect would be, that one has to wonder why we haven’t seen it published until now.

Secondly, one of the frustrating parts of being a statistician is that a 90% explanation of nearly every effect comes from some common-sense factor. So much so that it raises suspicions when a strong effect is shown along with a series of specific and fairly complicated ‘anti-explanations’. The article seems more interested in disproving the ‘idiocracy’ effect than finding some alternative explanation.