Just the opposite. There has been rising IQ in the west. It’s called the Flynn effect.
https://en.wikipedia.org/wiki/Flynn_effect
There are indicators of the ending of the Flynn effect and of retrogression, at least in certain countries. This new study would adds to the body of contrary indicators.
https://en.wikipedia.org/wiki/Flynn_effect#Possible_end_of_progression
This is mostly a phenomenon of “culture fair” tests. The tests use problems that are unfamiliar to all takers to remove cultural bias in the tests. The problem is that once people start taking a “culture fair” test its format becomes known and future takers score better than earlier test takers.
Vocabulary test results decreased somewhat during the 20th Century but they are ignored due to allegations of unfairness of these tests.