If this fairly nonstandard usage rule is significant enough to reply to someone to correct them on it, shouldn't it be listed in the guidelines?
The parent's usage is very common, so it seems a bit harsh for a mod to call them out on it, even if fairly politely, when they are not clearly in violation of anything they have been told.
HN is a "common law" community. Not everything is written down in a single place; the rules evolve as they're refined and reinterpreted by the mods.
I wonder what the best example of a "civil law" community on the Internet would be --- a message board where everything you weren't supposed to do was spelled out completely.
Civil law doesn't mean that everything is spelled out in detail. Ambiguous and vague characterisations in legal definitions are simply necessary to make a legal system work.
I think the real difference is that in common law "some other judges ruled this way a decade ago" is persuasive, while under civil law it doesn't really hold much weight.
The parent's usage is very common, so it seems a bit harsh for a mod to call them out on it, even if fairly politely, when they are not clearly in violation of anything they have been told.