Wondered about that as well, but another article, linked in the original one, mentions the original study dismisses that hypothesis, since it happens between members of the same family :
"In the new study, the researchers observed IQ drops occurring within actual families, between brothers and sons – meaning the effect likely isn't due to shifting demographic factors as some have suggested, such as the dysgenic accumulation of disadvantageous genes across areas of society"
https://www.sciencealert.com/iq-scores-falling-in-worrying-r...
"In the new study, the researchers observed IQ drops occurring within actual families, between brothers and sons – meaning the effect likely isn't due to shifting demographic factors as some have suggested, such as the dysgenic accumulation of disadvantageous genes across areas of society"