That's not what a slur is. The person saying it thinks it's a real category, and that you (and others) really fall into it. People have a right to think and even say things like that, whether you agree with the category or not.
They can't force you to accept that category, or use that language.
It's no different than a transman or transwoman trying to stop you for referring to them as their sex. You are allowed to think their ideology is bollocks, and use the category and language you think reflects reality. Even if they don't like it and feel it disregards their identity.
Trying to control people's language so it can only describe the world as you believe it to be is the exact mirror image of what TRAs have been trying to do. That is not bad just because they have an ideology that's not in touch with reality. It's wrong because people are entitled to their ideologies, and to describe the world in the way they think is accurate.
If cis was means to be inherently dehumanizing or diminishing, that would be a different story. That's what a slur is.
Honestly, sometimes it's not too hard to see the way in which the tactics of TRAs came out of the authoritarian left, it seems like enforcing rightspeak is baked in at times. There's no right not to find other people's ideas offensive or incoherent.