X-gender as a term has been around in Japan since at least the 1990's, first catching on in the Kansai region. If you think this is brand new and that Japan has been "sullied" by gender politics, you're either too young to know better or you were deluded into thinking Japan was a monolithic conservative paradise and just got a harsh overdue wake-up call. While we're at it, maybe we could discuss classic okama culture? Or visual kei pop stars? (and how one named Mana helped popularize gothic lolita fashion)
.... nah, let's stay on topic.
Hot take, but I like "X-Gender" better than "nonbinary". Nonbinary sounds like something a computer would identify as. X-Gender just inherently sounds badass, as almost anything starting with "X" does. It also doesn't center the concept of binary gender identity as something one has to be considered an exception from.
If the manga is this informed about the whole concept I'm going to show a lot more hope for it than others in this thread. In fact, I'll even bet the protagonist is the one who winds up exploring his gender identity a bit more. Sometimes all it takes is seeing someone live their personal truth to make you realize examining your sense of self was even an option.