The assumption doesn’t rest on “all” being atheists, but rather most are atheists, or at least not Christian’s. When you make a generalization, you are referencing a majority tendency, not an all or nothing. This is basic stuff. And I know not all of us are atheists. But most of us don’t believe in Christianity, so my “blithe assumption” actually holds because by the data, no where does it indicate that a majority of transgender people identify as practicing Christians.