I'm glad you like the demo !!! And no, in this case the pronouns are locked to "they": Zed is sort of a unique case. Whereas with my other games I give you a blank slate character, Zed is more of a character in and of themself: hence they have a set gender. It's also partially an experiment in gender-locked games, since so many exist for she/her and he/him protagonists but I don't think I've ever found one where you play a NB person (besides maybe a few coming out games specifically about gender).
Consider them like a character you play with rather than an empty space where you build a character to play. You'll handle Zed's reactions, their attitude, and their motives: some things, though, like their gender, a secret aspect of their appearance (which you do kind of control, but unconsciously; it'll be developed through your choices), and their goal (ie, the game would be rendered sort of moot if you wanted to play a Zed that hates their family member enough to not want to find them again) are the building blocks you'll use to develop them !!
This is partially because Hybrid was developed differently from the other games. Whereas the others I started with character customization in mind, Hybrid was the first game concept I'd ever come up with. I had planned on doing a straightforward Ren'py visual novel (this was before I even knew about other engines) and if I was going to get art, then I wanted to keep the protagonist static for simplicity's sake. I decided on NB because it's important to me, and like I mentioned I don't think I've ever seen a game where NB was used as anything but an option (if it even gets to be an option at all).
Overall, I don't think you'll notice a huge difference besides the pronouns. I don't really make a special point to ever differentiate between genders, even in my other games where you get the option, and this certainly isn't going to become a game about gender (god I would make such a terrible game about gender).
And I actually don't have any planned poly routes !! I'm only one developer and I find doing poly routes by myself requires more time than I'm able to dedicate to a project at the moment ;; I wouldn't want to just toss them in haphazardly for the sake of having them there, as I imagine it would sort of be insulting to people who actively seek those options out ;;
And thank-you so much for saying so !! I'm glad you like my writing <3