Could somebody help me with this? I'd like to understand it.
Isn't the answer always, for every metric ever considered, 'both'? The entire debate is a mystery to me.
The reason why I call this a mistake is that humanity doesn't work this way. At all. Humanity does have superpowers - i.e. language, opposable thumbs, supra-tribal social structures - but these are all widespread if not universal. Differences in skill are functions of specialization and training and disability is compensated for rather than shunned.
Some literature is smart enough to understand this and makes it very explicit that thinking your special powers make you inherently better than everyone else is a bad thing. The villain of the X-Men comics is Magneto and not Professor X for this reason. Harry Potter goes unusually deep[0] in explaining how wizarding society is cursed with all sorts of horrible fantasy racism, in ways I don't think a lot of HP fans have fully cognized[1]. You have the antagonist house Slytherin, full of wizards who coined the term "mudblood" to denigrate wizards who were born to muggle parents; the general shitty attitude that a lot of wizards (even Dumbledore) have to muggles; the fact that non-human sentient magical creatures are either treated poorly (goblins, giants) or outright enslaved (house elves); etc. Books that make this explicit avoid falling into the trap of magical fascism, but a lot of this literature just never bothers, either because the writers are bad at their job, or because they very explicitly believe the bullshit they're writing.
Yes, there's a leap of logic here. The original writer probably should have included some of the steps they omitted to get from "PSI powers" to "racial supremacy".
[0] Relative to J.K. Rowling's generally narrow and very England-centric reference pool.
[1] If they haven't already sworn off Harry Potter because, well, y'know... J.K. Rowling.
To be fair, this seems as much the fault of the author's. She seemed to have cribbed notes from a lot of smarter literature without truly thinking through the consequences of it. Many of her subsequent essays and writings seem to make it clear that she wasn't actually meant to be criticizing magical fascism tropes and is more inclined in real life to side with "magical fascism", and she may not have fully cognized elements of her own writing that attracted some of her original audience (many of whom are exactly the ones who have sworn off the franchise because of her subsequent actions, and those essays/writings).
Also, I think Stross overlooked another Campbellian thread that tied A. E. Van Vogt to Campbell's publications: A. E. Van Vogt wasn't as central to Dianetics as L. Ron Hubbard, but did play a major role in pre-Scientology Dianetics (per Wikipedia that was including early involvement in some of the "alternative accounting" tricks that kept the company/franchise out of a couple of bankruptcies and eventually led to Scientology though he was semi-retired by the time it started to turn into that).