[ithkuil]

I think your mixing up two distinct points here:

a) whether it's ok or not to insult person A with terminology that currently insults group B as a side effect. (The topic here would be "to PC or not to PC, have we gone to far with it etc; which is a conversation that can and should be had)

b) whether it's ok, while discussing point "a" above, to insist that some words mean something else than what it's generally understand in good faith, in order to apply ineffective whataboutism ("what about idiot, it used to be a medical term!") to the conversation. To what goal? Claim that you can use "retard" because you can use "idiot"? Let's not play etymological games, if you want to discuss about PC overreach, discuss PC overreach; there are plenty of good arguments to be had against extreme political correctness and censure that don't involve having your readers rolling their eyes at blatant attempts to clutching at straws.