If HN mods think the rule should be applied whatever the community thinks (for now), then yes, it needs a rule.

As I see it, down-voting is an expression of the community posture, rules are an expression of the "space" posture. It's up to the space to determine if there is something relevant enough to include it in the rules.

And again, as I see it, community should also have a way to at least suggest modifications of the rules.

I agree with you in "People who can't take a hint aren't going to read the rules". But as they say: "Ignorance of the law does not exempt one from compliance."