You raise a good point that does make me wonder: why isn’t there more pressure from free speech advocates to liberalize moderation on HN?
I think that probably stems mostly from Hacker News avoiding controversial subjects in general. When they do slip through, the moderation can be pretty bad, particularly when it comes to new accounts. I've seem innocuous comments shadowbanned for voicing fairly milquetoast heterodox opinions (with shadowbanning in general being a pretty unpleasant action). Usually that isn't an issue with accounts that have been here for a long time, but since Hacker News doesn't allow people to delete comments, you have to be comfortable with having that comment tied to you for decades to come (not always the safest thing in this environment).
The whole thing ends up exerting a chilling effect on alternative opinions.
We see this with Twitter now. People complain about various tweets being blocked, and there's always a "back-and-forth" about how Tweet A is against their guidelines but somehow Tweet B isn't. But when they outright ban (e.g.) Donald Trump, there's no ambiguity any more. The discussion moves beyond the minutia of spam / bot handling and into something more concrete.
Though you're right: censorship of those with smaller voices is at least as problematic. We just all ultimately have limited resources available and focusing on the more clear-cut examples is more likely to be successful.