I also gave up reading it, but from a few paragraphs below, when they said knowledge work they mean stuff like marketing, finances and other relatively high paying jobs that most people aren't passionate about but choose to do for money. I think it's a weakness of the article that it isn't upfront about what it means, so you're not wrong for looking it up, but yeah, they meant that.

The article itself seems to be on alienation and stress, but as I said I didn't finish it, as it's quite long and not engaging enough, at least to me.