One could claim that having read enough post-modernist work to be able to successfully emulate it, he had in fact become able to coherently express ideas which he could then also look at and proclaim to be nonsense.
Alternatively, given Sokal's own description of the paper as "a pastiche of left-wing cant, fawning references, grandiose quotations, and outright nonsense . . . structured around the silliest quotations [by postmodernist academics he] could find about mathematics and physics", one could argue that Sokal was merely repeating back ideas which post-modernists had already espoused, which leaves their resulting publication of his "nonsense" rather unsurprising.
Alternatively, given Sokal's own description of the paper as "a pastiche of left-wing cant, fawning references, grandiose quotations, and outright nonsense . . . structured around the silliest quotations [by postmodernist academics he] could find about mathematics and physics", one could argue that Sokal was merely repeating back ideas which post-modernists had already espoused, which leaves their resulting publication of his "nonsense" rather unsurprising.