There are two aspects to this. Firstly, postmodernism hardly concerns itself with venturing into scientific arenas at all. The endeavors which Sokal et al. deride are not representative of postmodernism as a whole.
Secondly, while I haven't read the papers from which Sokal et al. take their examples, and don't know what they were about, I am certain that they're not trying to contribute to scientific knowledge in any conventional sense. I gather from a friend who studies this stuff that Irigaray was playing with the language of scientific discourse in some way. I haven't looked into it further. Some of the stuff Sokal derides certainly deserves it. One of his examples is where Irigaray's work is used to justify sloppy thinking about feminism in science education. That is definitely problematic and deserves to be dismissed out of hand. But generally speaking, Sokal picked out these quotes without clearly explaining their context and intent, and that seems problematic, too.
Secondly, while I haven't read the papers from which Sokal et al. take their examples, and don't know what they were about, I am certain that they're not trying to contribute to scientific knowledge in any conventional sense. I gather from a friend who studies this stuff that Irigaray was playing with the language of scientific discourse in some way. I haven't looked into it further. Some of the stuff Sokal derides certainly deserves it. One of his examples is where Irigaray's work is used to justify sloppy thinking about feminism in science education. That is definitely problematic and deserves to be dismissed out of hand. But generally speaking, Sokal picked out these quotes without clearly explaining their context and intent, and that seems problematic, too.