I'm not sure whether you clicked on the link, but note that the line you quote was in response to a comment asking "David Graeber is an anthropologist […] What qualifies him to talk about the economy and the function of various jobs?". You're calling it a rhetorical trick etc: that may make sense if he had employed it in the book, but as a response to the specific comment he was replying to, it seems reasonable. (That's why I included "(in reply to https://news.ycombinator.com/item?id=22030745)" when quoting Graeber's comment.)