"Be kind. Don't be snarky. Have curious conversation; don't cross-examine. Comments should get more thoughtful and substantive, not less, as a topic gets more divisive.
"When disagreeing, please reply to the argument instead of calling names.
"Eschew flamebait.
"Please don't post shallow dismissals, especially of other people's work. A good critical comment teaches us something.
"Please don't use Hacker News for political or ideological battle. That destroys the curiosity this site exists for."
Now please consider how many of these guidelines the "You've been brainwashed by left-wing media" insult violated.
The comment was downvoted not because it "told the truth" or talked about a taboo subject, but because it violated the community norms of keeping the site friendly, and making substantial, thoughtful comments.
Now if the poster had omitted the insult and actually substantiated their claim about diversity not mattering much instead of making a throwaway comment about it, they probably would have gotten a bunch of upvotes and maybe even sparked a lively debate.
HN is open minded about intellectual inquiry as long as that inquiry doesn't challenge anything an average Californian already believes