The idea of the "existence" of a trans person is subject to debate. There are a significant number of people that believe that "being" trans is actually not something of substance, that it is "all made up". Being an immigrant is different, nobody questions that an immigrant that themselves attests to being from country A and is now living in country B is making that up.
Bostock was a ruling about sexual orientation, not about gender. I think if you are citing the 2020 PEW poll on this topic it is probably quite obsolete as it seems clear that the partisan valence of this issue has changed quite a bit very recently. And even before 2020 the truth is that disputing the legitimacy of transgender people is a low salience issue for a lot of people but very activating for others (much like immigration) so the proportional impacts of what seems like an unpopular opinion is inflated.