Exactly - you can only add so many inner matryoshka dolls before they're not big enough to sustain critical mass.
If HN is a subset of other sites (though arguably its approach to UX and moderation offer something different), a site targeting a subset of HN's users seems unlikely to get much traction.
The things this site used to offer was there might be an article about a security breach, a brand new highly anticipated feature in a major product, a historical article about a long forgotten but major technological achievement.
Then a key person from that story would chime in with their take about what the article got right or wrong, offer to answer questions. And that is assuming they didn't directly make the post.
Nobody actually wants to read self promotion blog spam, besides other people pretending to be engaged so they can do the same
Depending on where you're reading, it's Hacker News that's a subset of just about anything else.