Just based on empirical observations, in the hundreds of negative comments about “cisgender” as a vocabulary term that I’ve received over years of writing about LGBTQ issues, I’ve never once found that the commenter actually supported transgender people.
In my experience, objections to the neutral term “cisgender” have always served as negative rhetoric rather than as discourse about trans people as people.
I’m a white, gay, older, cisgender man. All those words describe me in certain accurate ways. They can’t and shouldn’t reduce to who I am as a person, but they certainly aren’t insulting.
As you say so well, rejecting the term cisgender is to reject the term transgender.