The article covers this:
> One potential explanation was quasi-eugenic. As in the movie “Idiocracy,” it was suggested that average intelligence is being pulled down because lower-IQ families are having more children ("dysgenic fertility" is the technical term).
> [...]
> However, a 2018 study[1] of Norway has punctured these theories by showing that IQs are dropping not just across societies but within families.
The studies would seem to contradict each other, which isn't all that surprising.