Retared as a slur is not very offensive because it would be particularly insulting to the recipient to be called mentally handicapped, it’s easy to shrug that off, it is offensive because it devalues mentally handicapped people. But I don’t expect you to understand this.
I am sorry but you sure cannot expect me to understand such nonsense. I don’t find mentally handicapped people of any less value, therefore to me, an instance of calling anyone mentally challenged (even if they are not) cannot devalue anyone.
If I call you an “astronaut” and you are not, does that devalue you or astronauts? No, because I don’t find your nor astronauts of any less value. It would only be devaluing to someone who thinks that specific group of people has less value (and your post indicated that you might be someone like that).