I'm not sure why you are interpreting the parent's comment as similar to "claiming that unions would actually hurt you". It's just a post talking about how management in companies works around the illegality of telling employees not to talk to about pay. It's in the first sentence.

It's not like the parent comment is defending the practice, just how it's done in practice to discourage employees. Yes, groups of employees that don't band together believe that it will hurt their own bottom line if they talk about pay.