By its very nature, postmodernism can't be correct or incorrect. The most it can do is provide a perspective or method of analysis. Some people might find it interesting or even useful. (Personally I see it as trite intellectual masturbation, but that's just me.)

It is very, very rare that a fringe or radical ideology that holds fringe views on almost all topics happens to hold a correct and important stance that is also very rare among non-fringe ideologies. Support for Palestinian rights has definitely not been as common as it should have been in the United States, but was still quite common among many non-fringe groups and individuals. You didn't, and don't, need to go to a radical Marxist critical theory publication to be better informed about that topic. I think the author makes an admirable defense of his magazine and shows it was not quite as crazy or worthless as the Sokal affair initially made it out to be, but we don't need to go all the way in the other direction.

Despite Alex Jones watchers all citing it as a great reason to watch him, you don't, and didn't, actually need to watch Alex Jones to learn about Jeffrey Epstein's crimes. Alex Jones was not the person who broke that story or even popularized it - and even if he were to have been, it wouldn't mean he was factually correct about most of it or about anything else.

Sometimes there are brilliant people with fringe, contrarian views or findings that initially get ridiculed and are later found to be correct or valuable. But Cantor was already a respected individual for his past work; he was not some no-name crank mailing theory-of-everything letters to mathematicians. Ramanujan was closer to that, but Ramanujans are so incredibly rare that you really need to be extremely cautious. And math is unlike almost every other field, where fringe claims can generally be objectively and independently confirmed or refuted, so you need to be far more cautious about nearly every other field out there.

However, I agree with your point that at the very least all sides should be initially heard out. Just not necessarily heard for very long.