"I'm sorry, I forgot you were retarded, my bad." I think it would be unfair to look to apply opinions on the above example to all instances of the word retard. It would be unfair for me to say it is totally meaningless. I've read that swearing can be split into five broad categories: emphatic, cathartic, idiomatic, dysphemistic, and abusive. I think a lot of people will take the above to be abusive, but functionally I only see it as dysphemistic. There is no truth to the statement. He knows that, the victim knows that, and everyone else knows that. It substitutes a lesser word but really causes no more harm, and in that sense I would say it's relatively meaningless. Anyone who takes it as a synecdochial attack on the retarded probably needs to be insulted to add some spice to their life. I do pity him for his boring insults though. It's going to be a tough life with the constant feeling of l'esprit d'escalier if he continues like that. Always remember - think before you speak.
Those categories would be from Steven Pinker's book The Stuff of Thought: Language as a Window into Human Nature. I can certainly see a dysphemistic quality to its usage, and I think there is also an idiomatic component to it as well. He's clearly trying to making a statement about political correctness when he uses it as a reply the person, further evidenced by the fact that he brings up "social justice tumbler etc." later on. You seem to reject the notion that there is an abusive quality to it, but I would argue that regardless of which category certain word usages fall into, particular ones like faggot, nigger, cunt, and in this case, retarded, pretty much always have some degree of abusive quality built into their contemporary usage, regardless of their intent. For reference, here are Pinker's categories with descriptions: Abusive Swearing - Abrasive words designed to insult. Idiomatic Swearing - Attention-grabbing, establishes informality and asserts ‘coolness’. Emphatic Swearing - Similar to idiomatic swearing, used for emphasis. Cathartic Swearing - A response to stress e.g. spilling hot coffee over yourself.Dysphemism - Intentional usage to elicit a strong emotion. cf. Euphemism.