The author starts by claiming he was surprised that he couldn't come up with positive things about a coworker, but then goes on to describe a list of grievances about someone who was clearly intolerable. He even mentions this later:

> If you’re currently doing them, stop. I’m not saying this because you’ll be insufferable (though you will be)

His initial "mistaken" judgment ("how did I get this so wrong? Am I just an idiot?") of the person in question just doesn't seem genuine in retrospect.