It is very possible to write about niche topics with gusto and at length, but without finding an audience or a circle of peers. Twenty years ago it was common to call this the "long tail," a term popularized by a bestselling middlebrow book.
I put my trust in the "long tail" of the Internet audience and wrote in public for a few years. The experience did not shape me intellectually, as reading a good book or learning a skill might have done. And it did not bring me into contact with a circle of peers who learned from each other.
The author even offers the example of his wife, who published a long piece of writing in a niche different to his and did not benefit in the "search query to find people" way.
I think there is a likelier and more specific explanation for the author's observations. Namely, over the past decade, guru figures like Scott Alexander have popularized a style of academic-sounding essay-writing that is diffuse, meandering, and often very long. Fans of this kind of writing also reward authors who aspire to global or timeless significance by writing about a set of "cornerstone" topics that includes Plato, Roman stoics and the Roman Empire in general, Tolstoy, Enlightenment-era American political philosophy, and popular takes on behavioral economics and evolutionary psychology.
Some of this guy's posts struck gold with this audience and got widely disseminated through their social networks. I strongly suspect that writing about historically and thematically adjacent topics (e.g., Russian Realists other than Tolstoy) would produce a lot fewer "search results" for the author.