The word was used correctly and not in an offensive manner: https://www.lexico.com/en/definition/retardation — perhaps you need to readjust your outrage detector?

I think the post was clear as to which meaning they were referring to. I would also argue that ‘slow’ is frequently used in the same way as one the one that you are concerned with.

Agree. If you have to clarify usage use a more precise word…