The whole reason they want 'cis' to be a slur is because they do not want to be named.

The unnamed is the default normal. If you have a label, you are a sub-category. To them, this feels like a downgrade.

But we are all lots of little sub-categories of humans - some with clear and well-known labels, some with fuzzy edges and differing uses. That's OK. There's nothing wrong with being a member of a sub-category of human. That is, in fact, normal. Because there is no singular 'normal' human.