Why not just say what you mean, instead? If the desire is "no replies that are or might spark a controversy", then why doesn't the rule say that?
Better yet, go all the way and forbid replies entirely. That achieves the same stifling of conversation, in this one context where it's deemed "terrible", without the enforcement that can seem capricious and arbitrary (as you say yourself, "it's often not easy to tell the difference") and can needlessly shame an otherwise well-intentioned commenter.
If you can explain in a short, simple sentence what the broader purpose of the rule is, then do so in the rule itself. Brevity may be the soul of wit but, but I expect a higher standard than rule wittiness from HN. The https://news.ycombinator.com/newsguidelines.html do this fine.
Wouldn't you rather have compliance than enforcement?
That's why both the HN guidelines and that Who Is Hiring rule are written in simple language that gets the bulk of the point—the spirit of the law—across, without pretending that there isn't still room for interpretation. Readers are expected to interpret them reasonably, and in practice this works just fine. Sometimes they interpret them differently from how we do, and then we try to explain better, on a case by case basis. It's ad hoc and imprecise, which is exactly how something as messy as a large internet forum needs to operate.
You're implying that the rule here is written like the guidelines, but it isn't.
The guidelines provide some kind of explanation, reasoning, or purpose adjacent to a rule.
The "try explain better, on a case by case basis" doesn't actually succeed, only the same reason, in, perhaps, a different word order.
Surely you don't need that kind of repetition of explanation of purpose for the guidelines, since it's already there to be read. Why such resistance to doing that here, too?