You seem to have some kind of misconception about eugenics, the thing that everyone agrees to be unacceptable is "improving" mankind by forcing some people to not marry or to not have children. The thing talked about in that thread is voluntary modification of ones own dna. Deleting comments simply supporting this would be crazy.
HN is not a crowdsourced knowledge base like stackexchange or wikipedia, to expect high percentage of accuracy on all posts, it is place of conversations and having people with different viewpoints makes it more interesting. For the thermodynamics example did you post the correct version after researching? Or maybe you found it too late when conversation have already moved on? Unfortunately that's the way all conversations work in real world too, best arguments come after it.
I am not saying that hn voting is the perfect system, but it is way better than the system where moderators check if comments are true or "right wing BS" because then instead of discussions you'd get echo chamber of everyone agreeing with moderator, and it's not guaranteed that it will be left wing echo chamber you wish for.