You are anonymous here, and your post contains no personal information, and none of your real life friends and acquaintances will see any information you intended to keep private.

With facebook, you are not anonymous, many of your posts would contain private information, and many of them could be pushed to people you didn't want to share with.

I don't know what threat models you find compelling. But the damage HN could do to someone with the info they have from an account holder is an order or two of magnitude less than the damage facebook could do.

It all boils down to: you're storing data in someone else's database. At this point, technically, they could do whatever they want with it. All that stops them from doing so is their ethics, the law, and the desire to keep their business.

That's also the only thing that stops tens of thousands of people from walking into my house, murdering me and leaving with my possessions. It is working so far.

What kind of social network could exist where no one besides the author can modify the content or privacy settings of their posts? I'm guessing every user would have to self-host their own content?

I suppose a blockchain based social network could boast that feature.

HN mods can edit others' comments? Can someone else confirm this? I can't think of a valid use-case for this.

You are failing to see the point. Any website could theoretically do anything they wanted to their own website.

Reddit does not allow mods or admins to modify comments from the website UI, but that didn't stop the admin from modifying comments by editing the database directly.