When people say "Trans Rights Are Human Rights", what they mean is that our fight is your fight too.
Shall people have bodily autonomy or not?
Shall healthcare be a human right or not?
Shall people have equal rights and opportunities regardless of their gender or not?
If trans people can't have rights, no one has rights, not really. They'll start with us, but it won't end there.
"If X people can't have rights, no one has rights, not really" is how fascism metastasizes. Once you decide it's okay to take a right away from a group, no matter how small, it's not a right any more — it's a privilege, to be granted only reluctantly, and only to the chosen few. In a few short years, the very concept of "human rights" is eroded and ultimately abolished.