That's like debating the definition of homicide or rape. There is no nuance here.

People absolutely do disagree and debate what is and is not rape, though. Legal definitions exist, but have loads of subjectivity. E.g. some argue that threatening to break up with a partner over lack of sex is coercion and thus rape.

Exactly. I think people socialized into certain conversational norms in politicized online spaces, ridiculously overestimate plausibility of the rhetorical gambit of going "gee, who's to say?" when attempted out in the wild.

I think one strength of the liberal academic tradition is that whether it's philosophy, whether it's law, you get introduced to the "whose to say" archetype early on and get inoculated against it. It's not just that the concepts are well enough established that they're resilient against such skepticism, but even in cases of uncertainty, routine amounts of conceptual uncertainty are not a deal-breaker to investigating and understanding urgent moral issues.

A real argument in the negative would be along the lines of "here's how food truck inspection policies are tied to well-established norms that better explain the outcome of famine than intent to destroy". A not real argument is spontaneous, mid-debate discovery of the transience of linguistic meaning, discovered just in time to skirt the question of genocide.

The trouble with this form of skepticism is it can only ever be hypothesized, never actually consistently embodied by real people. Long before navigating to hacker News, you would look at your computer and be paralyzed by fundamental puzzles like "what is electricity", "what is information", "is there really an external world" and so on. It wouldn't have been discovered mid conversation about genocide.

Homicide? Like abortion? No nuance?

Rape? Like age of consent being different across regions and time? No nuance? Like how half the planet laughs when a boy gets molested by his attractive teacher and the other half calls it rape?