For the philosophical belief to work, the people who seek to redefine themselves have forceably redefined all people.
It isn’t them saying ‘people with transgender identities’ and ‘people without transgender identities’. It is them declaring everyone has this thing they belief in - a gender identity. That is what this word ‘cis’ means.
But then it fails even more when it then also means in the term ‘cis woman/girl’ includes male people with particular medical conditions that mean their bodies go through male puberty to virilise.
Religious people can call other people non-believers of their religion and this is a material fact. It is real and it is meaningful. I doubt people would reject that. It is a null option.
’cis’ is not a null option. ‘Cis’ is not describing the lack of belief because it demands that everyone has a gender identity. It is enforcing belief on the recipient of that label.
That makes it rejectable in a way that the phrase ‘person without a transgender identity’ is not. And yes, of course everyone in a free society can reject any meanings of any words they which. We are talking about people using the words cis in the public sphere on this thread. Where those using the language expect to be understood and their message communicated.