Anyway I have no comment on the specific claim being made here, I just really dislike it when discussion is stifled by saying “I’m right and no one can ever disagree”.
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.