"to talk about disparaginly or negatively" is a verb form of "slur", the noun form is a little bit different. You can slur Elon Musk by calling him a privileged phony, but that doesn't make "privileged" or "phony" slurs themselves.

A more fitting definition of "slur" in the sense that it is used here is "a derogatory or insulting term applied to particular group of people". "Cis" is not a derogatory term, just like "trans", "white", or "Black" are not themselves derogatory terms. You can probably imagine some actual slurs that apply to each of these groups (tbh I'm having a hard time thinking of one for "cis"), but I don't intend to write them out here.