The OP's explanation is correct. The word "retard" being "universally regarded as an insult these days" is just straight up not accurate - on that sub or otherwise. I don't count myself as a linguist but have learned "get around in an emergency" in a few languages and have studied the history of language a bit, and it's very common for words to absorb new meanings in different contexts.

As others have pointed out, "retard" is an anagram of "trader".