The "dignitary affront" thing is frustrating because it reads to me as an attempt to emphasize/acknowledge how serious the slurs are, but because it uses specialized terminology without real explanation it comes across the exact opposite way. I'm familiar with the idea of "dignitary affront" in the context of a "dignitary tort" – a term usually used for something that is so severely harmful/offensive that you can sue over it, even though it's not a physical injury. (The most pertinent example here is probably the tort of intentional infliction of emotional distress.) So I read Appiah as trying to use that terminology to emphasize that hate speech is so much more severe than mere rudeness – but he's ultimately throwing in jargon from an unrelated professional/academic field that isn't helpful here.