The article clearly describes moderators watching videos flagged as offensive, and these videos included "amateur and profession pornography" and many that contained "child abuse, beatings, and animal cruelty". Furthermore the article discusses moderators having to deal with videos shot during the Iranian revolution in 2009, including the murder of a young woman, "a shaky cell-phone video captured her horrific last moments: in it, blood pours from her eyes, pooling beneath her."
The article presents many more examples of the enormously disturbing tasks assigned to moderators. Based on tens of years in clinical practice I'd consider it likely that viewing such material, especially seeing it frequently, can precipitate acute and chronic stress disorders in vulnerable individuals. I understand the point this is only hypothetical in the absence of systematically collected data, call it a clinician's hunch if you want, but still I'd put money on it.
Rereading the article confirms for me that it is an excellent piece of journalism, perhaps even exceptional in the current era. Moderators were exposed to far more than words, in a way similar to 911 operators suffering exposure to trauma without being at the scene. Forum moderation has very little in common with the subject of the article.
You are severely underestimating the effects of this kind of experience on people in such positions. You should learn more about stress disorders, the complexity of which is bound up with the unique and highly variable attributes of individuals. Some people are much more resilient than others making it misleading to generalize about how people will respond to given situations.
Most of all, I strong suggest you (re)read the story and if capable, allow yourself to empathize with the plight of the moderators.