I'll take you at your word that you're not trying to be clever, and that you're genuinely curious, and thus not interested in an argument that must be won. And, so, I'll do my best to elucidate in an attempt to satisfy that curiosity.

I suspect that the poster is referring to the whole of his experience as the pool of contexts in question, and thus can state that he knows of no one who is comfortable with the word choice - given that most people don't use the word "retarded" as the past tense of the verb "retard", which means to hold back or slow down, I'm inclined to agree.

Retardant is not in question. The use of retard as a verb as it's intended to be used is not in question. The vast majority of utterances of "retarded" are used as a pejorative adjective, so while I see your point, I think you're missing his: The reality is that the adjective form of retarded is, in nearly all common-use cases, a careless diminution of those with developmental or learning disabilities, as it serves to suggest the idea/thing/person could only have come from someone with such a developmental or learning disability.