People feel that "cis" is a slur because it puts them on equal footing with trans people.

They prefer to consider themselves "normal" while "trans" people the weird ones whom can only be labelled with anything other than "normal." Being called "cis" makes them feel as if they can also be labelled as something other than "normal."

As non-trans person, I can partially confirm. First time when I heard it, my reaction was "why do you need a new word for that?" Now I kind of used to it, but still there is some amusement each time I hear this word - it seems unnecessary, when "non-trans" would suffice if needed to avoid confusion. But slur? That's nonsense is only in Musk's head.