My actual answer is in the last line, but I'll ramble a bit first. You make an interesting point, but I think there are times in which we can prohibit certain types of posts and comments that might technically be censorship (using the definition of suppression or prohibition of things that are considered obscene, politically unacceptable, or a threat to security), but they aren't bad censorship. I would place some sort of test using focus and/or ubiquity.

Focus: if a community has a stated purpose, then it's okay to censor things that don't contribute to the purpose. If I ran a forum dedicated to ham radio, then it's okay to censor topics about NASCAR. Usually forums have a miscellaneous section for off-topic discussion rather than prohibiting it outright.

Ubiquity: if a community is small enough or in a limited setting, it is okay to censor things. It is accepted practice in many households to censor profanity. That's okay, because each family's censorship only applies to a few people. It is okay for a school to censor certain disruptive things (some people may disagree on what is okay to censor), because it's limited. Once a kid goes home they can read a banned book if they choose to.

Unfortunately for YouTube, I think they fail on both the focus and ubiquity tests. YouTube's purpose (from a layman's perspective) is to share videos. If someone uploads a video then they are staying within YouTube's focus, and YouTube has been pretty free with allowing any video as long as it isn't illegal. So they shouldn't be censoring crackpot videos, because they aren't illegal and seem to stay within YouTube's norms. [Some economically minded people might argue that YouTube's true purpose is to sell ads and should censor videos that cause fewer ads to be shown, but YouTube's previous behavior is that they demonetize/pull the ads from controversial videos, not deleting accounts]. And YouTube has clearly crossed the ubiquity threshold, with zillions of views all over the world.

I struggle a bit with whether YouTube should be legally required to follow the First Amendment (they are a US corporation, not a government), but I can say that I think that their deletion of David Icke's account leaves a bad taste in my mouth- not because I like him, but because I worry that YouTube may one day bow to majority pressure and censor a topic in which I belong to the minority.

To get back to your actual point- HN's guidelines are technically censorship, but they are good censorship. It's designed to keep a community focused and to foster good discussion. The best part is that HN's guidelines are easy to understand and seem to be applied consistently. It would be great if YouTube adopted easy to follow policies and would terminate accounts based on clear breaches of policy, rather than just playing whack-a-mole with whatever topic is hot at the moment. There are tons of other videos spreading misinformation on YouTube about other public health topics (the anti-vax movement being a big one), why is David Icke's account being deleted and not theirs? There are tons of other conspiracy theorists on YouTube, why are Alex Jones and David Icke the only ones to get their accounts deleted?