I'm questioning the suitability of "retardo" as a rebuke term in this context because it equates mere ignorance or stupidity with a condition (mental disability) that for most of its sufferers, is not reversible.

Your killing me here man!

So the world should bow to your judgement about suitability?

You've agreed, that there was no intent. You've agreed, that there was no association in context. Just as I hope you will not interpret the first line of this response as an accusation of attempted murder; you must see that neither person wrote with any intent to disparage the mentally handicapped; and nor will the majority of onlookers take it to mean that. No more than when we hear people say "you/he/she are crazy!".

Proscribing the use of particular English words because they have more than one meaning, one of which would be derogatory to some group or other, even when obviously used for the other meaning, simply makes light of the real problems faced by those groups.

At this point, you've made and reinforced my point exactly, all that's left to do is point you at some appropriate previous discussion.