I think the contents of the warning explain the warning on the top adequately. There are some links in there as well, and they in turn lead to examples and citations.
Considering the amount of visitors on HN, it's possible you've never seen or met anyone matching the described behaviour (in the linked pages). But the people on the receiving end might not want to warn for the thousands of people it doesn't apply to, but rather the ones where it does.
Say it's only 10% of 1000 visitors matching the HN referrer, that's still 100 people against a handful of volunteers. If only 10 of those think it's funny to SWAT them, or send them things they didn't order, all day, every day, that's enough to warrant such a warning.
I don't think that's the clue here. The clue is calling out such behaviour.
Maybe that's already something that people do, and the banner does nothing. Or maybe it's not something people do, and maybe some people start doing it, and then the banner does something.
The article doesn't, but it provides the context on why the warning does, which is why it is there.
That should answer your question: "What’s with the warning on the top?"
Perhaps you expected a list of all sorts of interactions between this website and that website, but I doubt the project volunteers kept a ledger to share that with you and that in itself would incite reverse brigading which solves nothing.
The project has apparently been harassed by some other group I’ve never heard of until today. I still fail to see how that has anything at all to do with HN. The wording of the warning definitely implies they’ve been hurt by HN.
That's because they have been hurt by HN. Not intentionally by HN itself, but they are also not the only target.
As for how you haven't heard of them before, that is a bit surprising. They have been mentioned here plenty of times.
HN being user-driven means they are an amplification and legitimisation vector for anyone who figures out how to abuse it. It's not an easy problem to solve, and I don't think any organisation (commercial, or otherwise) has really found a way to deal with this.
As a human side-effect, most people will either have to have experienced it themselves, or have this as part of their job to understand the issue at all, and will either deny it or assume it's exaggerated. Even those that are on the instigating side will not have a concept of the impact until it inevitably happens to them (as is common in that ecosystem).
Considering the amount of visitors on HN, it's possible you've never seen or met anyone matching the described behaviour (in the linked pages). But the people on the receiving end might not want to warn for the thousands of people it doesn't apply to, but rather the ones where it does.
Say it's only 10% of 1000 visitors matching the HN referrer, that's still 100 people against a handful of volunteers. If only 10 of those think it's funny to SWAT them, or send them things they didn't order, all day, every day, that's enough to warrant such a warning.