It's all about context and intent. In the context there is no intent to apply or relate the term retardo to anyone suffering from a mental handicap.
Yes, context is important. However, the "no intent" part is not true. The original quote (which, in the reply, was not put in quotes) is meant to deride a non-mentally disabled person to say that he was acting as if he was mentally disabled. That's the meaning of the quote and the oomph is given to it by the hidden disdain we have for people not as put together as us.
By seeking out offense, and choosing to be offended, on behalf of others, where no offense was intended, nor can be rationally inferred from the context, you don't help a problem -- you are the problem.
He did not seek out offense. He pointed out the offense that was already there. He did it by asking a question, which instead of answering, you attacked him, ad hominem. If you don't want to answer the question, or even if you think it is off topic, just say so, or don't answer at all. But don't attack him.