edited to add tl;dr: Siskind seems ticked off because recent papers on the genetics of schizophrenia are increasingly pointing out that at current miniscule levels of prevalence, even with the commonly accepted 80% heritability, actually developing the disorder is all but impossible unless at least some of the environmental factors are also in play. This is understandably very worrisome, since it indicates that even high heritability issues might be solvable without immediately employing eugenics.

Also notable because I don't think it's very often that eugenics grievances breach the surface in such an obvious way in a public siskind post, including the claim that the whole thing is just HBD denialists spreading FUD:

People really hate the finding that most diseases are substantially (often primarily) genetic. There’s a whole toolbox that people in denial about this use to sow doubt. Usually it involves misunderstanding polygenicity/omnigenicity, or confusing GWAS’ current inability to detect a gene with the gene not existing. I hope most people are already wise to these tactics.