reason why they don’t “gatekeep” harder anymore...

In a nutshell. They're fair-weather friends. This kind of catch-all affiliation lasts up until the group gets what it wants. Then the gatekeeping begins as they squabble over which of them actually gets it.

obviously something that 19th-century eugenicists and literal Nazis could only have dreamt of

Tribalism is tribalism. Look to Africa. With little to no intellectual basis for eugenics, they have spent millennia visiting inhumanities which would make the Nazi's blush upon whichever group drew the short straw that time around.