Because, IMHO, it more sense to say that the Rambam is talking straight
Do you really, in your heart believe that, if the Rambam were alive today and you went to him with a shaila about calling your children darlings, that he would say you were condemned to gehenim for it? When so many other actual sins don’t have that statement attached to it?
Or do you think that something was qualitatively lost in the translation of nicknames, and that the Rambam is talking about a case where the nickname can be destructive to a person’s life?
How much credit do you give to the Rambam?
PS – What makes names like Eeees a “nickname” vs. a special additional given name?