Musk himself has identified as such: “For the record, I am a fat retard.”

So, that is in fact, his word.

If someone refers to themselves by a particular slur, that does not grant you any social leniency to call them that too. Consider that exact situation with any other particular slur.

Musk using a slur about himself doesn't grant permission for others to use it. Would monegator have used the n-word if Musk had used it to describe himself? Hopefully not. And let's be honest with each other: Musk says things like this for shock value, because it's not an acceptable word.