Idk if I'd go so far as to say that it "objectively is." Inasmuch as there is a singular way of defining romance, Sorawo's relationship goals are missing several of the important features for the platonic ideals of romance.
I can't find the exact post I'm thinking of (or I'm hallucinating several posts & ideas together), but I once read an essay discussing & dissecting the ideas of what, exactly, a romantic relationship is, and what separates it from other concepts, with the goal of using that to provide a coherent definition of romantic attraction. I think it was by rotten-zucchinis? Or possibly by asexualagenda, this post of his is a reaction to that discussion and links a lot of stuff I remember around then.
Essentially the idea is that, while there's no 1 definition of romance that is perfectly coherent and universal to all things people call "romantic," there are several key concepts that are being gestured at by the term. No romantic relationship fits all of them, but when people talk about romance, those are the ideas they're communicating.
One of those is the escalator relationship. Romance is a relationship that "progresses" from friendship to dating to sex to marriage to children. The Nue is explicitly not that, that's the one that's most been discussed within the story. Sorawo's not following that path, and Toriko is no longer doing so as well.
Another is the nebulous idea of a shared life. Things like cohabitation, shared finances, "in sickness and in health." The Nue is, obviously, still a fairly young relationship, not at a point where those are serious considerations, but you can't tell me that you can look at the way Sorawo reacts to the idea of "family" and say she'd describe that as what the Nue is to her. As yuriskies says, that's probably the exact thing that Sorawo is running away from, whether she's articulating it to the audience or not.
The potential definition I remember most prominently from that essay was the idea that romance was a relationship that had the "potential to be sexual." Regardless of the couple's actual sexual status, a romantic relationship is one where, interpersonally and societally, sex is a real consideration. This is the only definition where the Nue maybe fits. Like, they settled on the name and the idea because of their first sexual experience together. Obviously, that first experience was much more supernatural than most sex, but it remains that the difference between Accomplices and the Nue is, partly, the sexual component.
9 & 10 are playing with that tension - Soratori are in a space where it's easy to read them as romantic, despite Sorawo's protestations. The reader can easily push them into that gravity well. But don't mistake that tension as "objective." Just because you've encountered a bear wearing a human face does not mean it's not a bear.