I've seen the word 'retarded' used in the exact way I used it above in many places and I've checked the dictionary and it has a simply secondary definition ('Dumb, foolish') which is roughly what I intended to convey.
Since giving offense is not one of my pet hobbies I edited my comment but had the situation been reversed I would have definitely afforded you more leeway.
To me 'retarded' did not seem rude or derogatory until you pointed it out, judging by the number of other people that are actually native speakers that use it in that exact same way it seems to me that you have your work cut out for you.
As for having my work cut out for me, I definitely expected this to happen (maybe not to this scale). HN tends to attract the kind of person who thinks that anything is fair to say any time, regardless of who it might hurt, because HN tends to attract people of massive privilege, many of whom have never been the victim of societal discrimination.