I am unlikely to see this film and so probably shouldn't comment.

However, I have a ( much loved) nephew with Asperger's and the pejorative insult on this side of the pond was not "retard", but "mong", I assume, a contraction of the term which used to be applied to

Down's kids, "mongol".

Either word should IMO not be in the vocabulary of any decent-thinking person, as IMO vilely insulting, and very, very hurtful.