In the context of talking about people, that word has everything to do with the people who it has been used as a slur against, including, but not limited to "mentally handicapped".

OP was clearly using the phrase as a derogatory term for people, and the only dictionary sense that fits there is the one that has ableist allusions. If the context of use were different, we wouldn't be having this conversation. For example, I wouldn't have a problem with the phrase "The PCM responds by retarding ignition timing—either until the knock disappears, or until maximum spark retard is reached." or "The Friar's alibi finds him at the right place but always a moment in retard".

That you're taking such a literal reading here makes me wonder whether your comment was made in bad faith such that I shouldn't bother wasting my time, but I'm hoping that there could actually be some meaningful dialogue here (after all, there's a reason why I didn't just report OP and move on). It might not affect your opinion, but I have direct experience of the r-slur that has been directed at me (not infrequently) when I am people read visibly disabled. I'm not "mentally handicapped", but as a word, it has grown far beyond it's original context of use. I say this to give context on my original comment — I'm not just going about tone policing people for fun: I commented what I did because it hurts to see that word thrown at people when part of what makes it effective as an insult is its attachment to people like me.

Once upon a time, the r-slur was actually considered one of the more appropriate words to describe people who are intellectually disabled. If I were alive in that era, I'd have likely been left to rot in an institution, and allowed only a fraction of the independence I'm able to have nowadays. But times change, and so does our understanding of the baggage that words pick up.

To draw an analogy, it wouldn't be appropriate to call a black person the n-word, on the basis that it derives from the Spanish word for "black". That etymology isn't wrong, but it's still missing the forest for the trees.

This my friends is a distinction without a difference. Dude really implied that being "delayed", as in developmentally delayed, has nothing to do with the handicapped. Astounding.

It has everything to do with "mentally handicapped" people. The word "retarded", used as an insult, derived from the term "mental retardation", which was previously the actual clinical and legal term for a person with what we now refer to as "intellectual disability". The use of "retarded" as a slur/insult is the whole reason why the clinical term was changed. It had come to be seen as derogatory and ableist even when used in a clinical context.

Source: the Diagnostic and Statistical Manual of Mental Disorders. The term "mental retardation" was used through the fourth edition, which was last revised in the year 2000. It lasted until the 5th edition, which wasn't published until 2013. Various organizations/agencies changed their terminology prior.

Fwiw, I'm a licensed clinician and I have diagnosed people with intellectual disabilities. I understand your perspective on the word, and I even shared a similar opinion until I learned how it has been used as a slur toward people who do have intellectual disabilities and developmental delays. Because of learning that, I now don't use it as an insult. We do better when we know better.