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:

Dysphemism - Intentional usage to elicit a strong emotion. cf. Euphemism.

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.