I have read the same thing. But my nonscientific sense is that our culture is getting dumber each year. Just look at TV and newspapers. Newspapers were written for 12th graders 25 years ago. Now, they are aiming a few grades lower. So my conclusion, again nonscientific, is that the uptick in IQs is due to a raising of the left hand side of the curve.
A few years ago I tried reading an unabridged version of Edward Gibbon’s “The Decline and Fall of the Roman Empire”, not taking into account that it had been written in the 1700s. Oh my head...trying to follow the complex English that Gibbon used was rough!