Musk didn't declare it a forbidden slur. What he declared forbidden was "repeated, targeted harassment" and that cis will be considered a slur by Twitter in the context of assessing whether something is "repeated, targeted harassment". Personally I think that's fair enough. There are countless words which can be slurs when used to harass.
If a transgender woman asks to not be referred to as a man, I would argue that use of male pronouns can constitute a slur. It can be a slur even if it's technically correct that they are still a biologically male person.
Calling an openly gay man "a homo" is widely accepted to be a serious slur, even though it's a term derived from Latin and the factual imputation conveyed by the word isn't in dispute.
It doesn't matter if a person happens to fit the technical definition of "cisgender" because slur-ness is never about technical correctness, or whether a word has Latin roots. It's about the reasons why the word was chosen and the purpose it is serving in the conversation.