In other words, they claim a common law trademark on it [1], but they have not registered it with the USPTO. This means they probably have some trademark protection, at least in some states, but not nearly as much as if they had a registered mark. For example, they might be able to get an injunction against a similar user in some states, but probably not damages or attorney fees. With a registered mark, they would be able to go for damages and attorney fees.
[1] See reply to you from 'disconnected' for link to claim.
I dont think there needs to be a trademark to not want to cause confusion. The majority of people in our field know that this is hacker news, sometimes they talk about it to someone who doesn't know. That new person finds "the hacker news" and thinks that their coworker is a 1337 hax0r.
https://www.ycombinator.com/legal/#trademarks
Hacker News is trademarked.