I've never taken their argument to be about grievance studies relative to other fields, but just to try to show that the field (and I'm not sure what a better term would be, given how many I've heard) is not concerned with intellectual rigor at all. This isn't them saying "people mess up sometimes and this invalidates entire scientific fields," they're instead saying that at least the journals they tested are not rigorous or intellectually consistent enough to be worthy of being taken seriously.
Or as Yascha Mounk (a Harvard lecturer on government and self-described lefty) put it:
For another, it is nonsensical to insist that nonsense scholarship doesn’t matter because you don’t like the motives of the people who exposed it, or because some other forms of scholarship may also contain nonsense. If certain fields of study cannot reliably differentiate between real scholarship and noxious bloviating, they become deeply suspect. And if they are so invested in overcoming injustice that they are willing to embrace rank cruelty as long as it is presented in the right kind of progressive jargon, they are worsening the problems they purport to address.
It would, then, be all too easy to draw the wrong inferences from Sokal Squared. The lesson is neither that all fields of academia should be mistrusted nor that the study of race, gender, or sexuality is unimportant. As Lindsay, Pluckrose, and Boghossian point out, their experiment would be far less worrisome if these fields of study didn’t have such great relevance.
But if we are to be serious about remedying discrimination, racism, and sexism, we can’t ignore the uncomfortable truth these hoaxers have revealed: Some academic emperors—the ones who supposedly have the most to say about these crucial topics—have no clothes.