It’s real insofar as there are people who subscribe to the idea and practice it. In the same way that Christianity or Wicca are real. The debate isn’t about this.
But because it is real in this way, it’s also real that there are people who don’t believe than one can be non-binary. There is more than one schema for gender currently in Western society, and these schemas are in competition for instance the traditional binary vs various different non-binary conceptions vs so-called “truscum” vs xenogender…
We know that gender itself as a matter of social practice (the work of second-wave feminists proves this definitively) so therefore all gender schemas are subject to social debate.
Expect, and this is where the crux is, that some (but not all) trans and non-binary activists are now claiming that gender identity is innate (while gender remains a social construct) and that in fact it is sex which is arbitrary and that in fact it takes a whole team of medical personnel to ascertain sex (the so-called “assigned at birth” model).
The broader point I am making is that because current Western gender schemas are in such flux, you can’t simply say that someone is Nazi just because they subscribe to a different schema than you do.