What clicked for me in this article is asking why I ever thought the internet was about ideas, what it might have meant pre-internet for people to be free of a "mainstream" in the old music and style subcultures, and what the consequences of "eternal september," really were.
When we say mainstream, I think we mean aligned to the dominant power, and the culture that comes with a mainstream is based on litigating rules for incremental advantage. Internet trolls use the higher bar for entry of tech focused platforms to harass people who are competing in the desperate political struggle for approval that is mainstream social media. Conversely, this mainstream culture is also the source of the visceral disgust some people have for lawyers, journalists, activists, and other people who affect being underdogs so they aren't held to the standards of honest people, and then antagonize others using those dominant rules as a cover. The same can be said for trolls. What it comes down to is this calculus of self permission to act like a piece of shit because you identify as the underdog.
It's not the platforms, it's the single idea that identifying as an outsider, resister, pariah, defender, or challenger, justifies cruelty because being "good," is defined by your chosen bugbear. The reality is the mainstream is not "good," the public sphere is an open sewer, and the only things that float in it are turds. It's not a right/left thing, it's a conflict between people competing for mainstream power and people trying to find freedom from that mainstream's domain, with some of each self-identifying as beneath decency.
The conspiracy-sphere of alternative internet platforms isn't the problem. I would defend the necessity of their existence because trying and failing and building new things requires real privacy away from the mindless predation of both the mainstream and the pariah net. Sure, take them down as a forcing function to make the new private sphere stronger with a higher bar to entry that is cryptographically enabled, but it's not solving the problem they think.