And it's not just a slogan!
It is the literal truth. Nothing, nothing that trans people want is something that isn't already a right for everyone else. Here in the US, we're supposed to all have equal protection under the law. So it isn't just that trans rights are human rights, they're enumerated rights, rights that are part and parcel of the constitution itself. So they should be doubly protected.
Trans rights are human rights, period. Anyone that doesn't get that probably doesn't get the point of human rights in the first place, or (worse) is rejecting the entire concept of human rights.