You can’t disentangle “smart people have few kids, dumb people have lots of kids, ergo we are gradually getting dumber” from eugenics (or class for that matter). It’s inherent in the argument. It’s also not how genes work.

It’s a very classist, flawed argument. In some way it’s emblematic of what the right calls “East Coast elitism” yet I find people from across the political spectrum have no issue embracing the argument (because they all assume they’re part of the “smart” group). “Wow look at all these dumb hicks having kids while the smart, responsible, educated people aren’t.“ You really don’t see the issue with that?

There’s even an XKCD for it: https://xkcd.com/603/