Did I miss it or did they not really address the claims in the section that quotes: "For example, Gladwell (2008) wrote that “once someone has reached an IQ of somewhere around 120, having additional IQ points doesn’t seem to translate to any measurable, real-world advantage” (pp. 78–79)."

They quote a bunch of people who say, basically that IQ above 120 hits diminishing returns hard, but they very quickly shift to a different argument, instead proving that rising IQ doesn't actually make things worse. Which is a different thing. So, are they trying to pull a fast one?

I've no great love for Gladwell, I just think that seems a reasonable claim that they could have blown out the water if they had data to disprove it, but they appear to have shied away from it.