That rule would be the first rule in the guidelines:
Off-Topic: Most stories about politics, or crime, or sports, unless they're evidence of some interesting new phenomenon. [...] If they'd cover it on TV news, it's probably off-topic.
Without commenting on this specific article, if there's one thing I learned from reddit, it's that heavy moderation >>> "the users choose and everything is free".
An example of the latter is pretty much any big subreddit you think of as awful and full of nonsense. Some examples of the former include the most well-respected science and history-related subreddits. I would honestly prefer there to be heavy moderation on HN with someone who has a vision for what "HN content" should be (even allowing for the fact that those people may disagree with me) than a free-for-all.
Random sites on the Internet are not the entire Internet, nor are they the government. I believe in Free Speech when it comes to people with power over me, not so much when it comes to websites that I can choose to leave at any time.
Anything that gets voted to the first page IS de facto HN content, as voted by HN users, unless it specifically violates some rule.