Tomboyish girl use ore too.
And it's pretty clear that if she is a girl, she is a tomboy and thus would use "ore".
It's like "watashi" or using your own name. First used mainly for girl, second for kid. But a girly boy will use watashi. And a kiddy grow up will use their own name.
Even more in japanese, Pronouns announce how someone present themselves, not what they "truly" are.
well... I'm simplifying a lot here for brevity's sake, but to speak of just the three most well-known pronouns, "watashi" is gender neutral, "boku" is masculine but with a youthful connotation, and "ore" is very manly. you're exactly right: pronouns are about presentation - especially when it comes to fiction, since they're a tool authors can use to communicate something to us about a character. in this case, nishi gave nova the "ore" pronoun to say "you should think of this character as being very masculine." in other words, if it turns out that nova is a girl, it will be a twist that defies the audience's expectations, not an ambiguity getting cleared up
just sayin' that no one should be too terribly surprised if words like "he" and "boy" get used in upcoming chapters since the pronoun choice tips things heavily (even if not definitively) in that direction