I don't think that's true. HN is not about some specific subjects.

We discussed everything from the Iraqi war to brexit to programming languages, etc.

While true, I think it has more to do with the way the relative niche-ness of HN shapes the community. The average person who comments on an Iraq war thread in HN is not selected from the same sample as the average person who comments on an Iraq war thread across all fora. HN consists of enough niche topics that the average person doesn't want to hang out here and make an account just to jump in on random Iraq war or Brexit threads (the current frontpage has about 25/30 tech-focused topics, many of which are beyond esoteric for most people). This shapes the community in a way that affects the make-up of the commenters on even such politically-charged threads.

Right. And if there is a thread on the Iraq War or Brexit, I'll pretty much guarantee that it's going to be heavily weighted with arguments and comments--mostly reflecting the talking points of a particular political perspective--that have been made a thousand times before. Plus a good smattering of "F so-and-so."

I agree with your conclusion, but the common thread I see is more the type of person than the subject matter. I don't mean that HN's audience is homogenous across some simple metric, but even among its vastly different members I see a shared respect of logical reasoning and evidence-based argument. That, and just generally smart people, are the hallmarks of the culture here for me (at least the positive part of it).

Right, and the subject matter (at least at the outset) has a large influence on the culture that develops. Challenging subject matter tends to attract people who share those traits.

Niche subject matter is a barrier of entry allowing only a specific subset/types of person in. To be precise a certain type of personality self-selects to find a site like HN.

That fact alone makes a significant impact on moderation.