The explanation is that IQ measures the cerebellum, and the declining nutrition and increasing brain damage forces people to rely more on the cerebellum, the statistical engine, instead of abstract thinking.
Though that goes against Europe hitting the decline earlier than the US which leads in sugars and unhealthy foods.
I suspect they don't do the research for real, they do some kind of simulation, and write it was in mice. They do know the metal changes the protein, but they incorrectly claim that the version without it works, but in reality the one with it does. They could'n make such an error if they actually did the research for real.