IQ by definition averages to 100, but if you compare present populations to past populations without the normalization there's a significant difference.

I'm no expert but my impression is that it's a widely accepted finding in psychology.

The average IQ is 100 by definition and constantly adjusted, yes, but you can keep track of which way it's being adjusted.

My fairly uneducated guess would be improved nutrition and reduced disease burden are big factors.

As a sign of how far the Western ruling class has gone down the rabbit hole, though, note that Flynn's own take on the broader context is now beyond the pale: https://quillette.com/2019/09/24/my-book-defending-free-spee... .