American IQ scores have rapidly dropped, proving the 'Reverse Flynn effect'

This blog post asserts that IQ scores didn't drop for the population as a whole, and that the drop for each individual group is due to changing composition of that group:

https://emilkirkegaard.dk/en/2023/03/new-study-didnt-really-...

For example, if the % of people who do a postgraduate degree goes doubles, it's no longer such a select group, so you'd expect the average IQ of postgraduate degree holders to go down. This doesn't mean IQ scores are going down for the population as a whole.

One more thing: why do so many papers that present charts that show how a mean or median changes over time, without also presenting charts that show how the distribution has changed over time?