Cis means on the same side of (some kind of scientific language).
You will be told that if your inner identity correlates with your sex, you are cis.
And here is where the difference lies.
Trans people push the idea that gender is an internal soul/lady brain/essence.
Whereas feminists, gender critical people, and actually even medical websites, trans websites etc, will tell you it's a set of stereotypes, dictated by society.
There really is nowhere, including official trans websites, that supports the idea that gender is an internal soul. It's the way you feel, sure, but based on what you see as stereotypes.
So you can only be cis if you feel as though society's expectation of your sex is correct.
I do not agree that I should do all the housework, write the Christmas cards, be paid less, be objectified, available for sex on demand, etc.
That's the technical answer.
"No sorry, my inner perception of myself does not agree with the gender 'woman', because gender is socially constructed by society, and disadvantages me".
The other answer is I'm dammed if I'm going to be defined in relation to less than 1% of the male population.
I'm a woman. A adult human female. It's simply a biological description.
I am not going to be defined in relation to a man who thinks he's a woman.
Also, you could ask them to define gender. It's always quite interesting. Because they will say it's your inner feeling of whether you are a man or a woman. But ask them to describe that.
Ask them, without referring to biology, to tell you the top three things that makes them think they're woman/man. It's impossible to do without resorting to stereotypes.
Saying something like I feel nurturing, doesn't cut the mustard. Because many men feel nurturing, and plenty of women don't.
If fact it's quite an interesting thought experiment to do on oneself.