Hacker News is still a centralized forum, owned by a single company and run on a single server. Being niche doesn't make it decentralized, it's just as centralized as Reddit.
I'm not aware of an actually decentralized equivalent of what Hacker News is (basic threaded text forums) but that might be an interesting project for someone to build on an existing protocol.
the second link goes directly to the largest instance. It has all the same federated interoperability features that Mastodon or Bluesky have, but with a thread/topic format instead of a microblog format.
Decentralized means there are many centers and if you don't like one center you can go to another one - the system as a whole has no center. You are here on the non-reddit forum system, which has no single center, but rather one or more separate centers for each discussion topic, unlike the reddit system, which has one center.
You may be thinking of distributed systems, like blockchains, which have no centers at all. Nobody is using those for social media yet.
If your thesis is that simply having separate threads with different topics counts as being decentralized, then Reddit is decentralized because it has different subreddits for different discussion topics as well as different threads. And literally every other social media platform counts as well, for similar reasons.
The "center" that matters in this context is the server and owning entity, not the taxonomy of the content. Decentralized platforms aren't owned and operated by a single entity, centralized platforms are. You can't just spin up a new instance of Hacker News with different moderators and just federate with it, because "Hacker News" as a network is entirely owned and controlled by YCombinator. Not so with, say, "Mastodon."
Also yes unfortunately there are already multiple blockchain-based social media platforms.
No, you are on a separate SITE with different topics. One or more sites exist for each topic, run by different people. They all compete, with no obvious single center. You are on Hacker News to talk about technology, mostly software. To talk about mostly networking, you could go to #networking on Libera. To talk about raising fish, you could go to I don't know where. Let's stick to those two examples I do know - why do you think that Hacker News is "the center" of the non-Reddit forum ecosystem, and #networking on Libera is not "the center"?
I'm not aware of an actually decentralized equivalent of what Hacker News is (basic threaded text forums) but that might be an interesting project for someone to build on an existing protocol.