edit: accidentally removed the quote i was commenting on when editing in Stross' comment, here it is again:

a belief in psi powers implicitly supports an ideology of racial supremacy, and indeed, that's about the only explanation I can see for Campbell's publication of the weirder stories of A. E. Van Vogt.

Maybe it's me but I don't think that is so self evident a claim to be posited without further explanation.

Best I can come up is he means the necessary implication of having superabled people in a fictional setting is that you have a de facto racial elite, even if the concept rarely breaches the surface of the text, like in the unfortunate sequel to the Dark Knight Returns by Frank Miller.

Edit: he addresses it in the comments (can't find a way to direct link from phone, its comment #14) I wasn't far off:

If you're a glutton for punishment, (re-)read Slan by A. E. Van Vogt.

Secret superrace with super-mind powers! It's totally a meme in vintage SF (goes back at least as far as Bulwer-Lytton's The Coming Race in the 19th century) and you rapidly end up with eugenics and breeding for desired traits (eg. psi powers).