This is a classic example of someone projecting their values, politics, and fear onto a technology and article with only minimal reading. That's not how Mastodon works at all, and anyone who's been there can say there is plenty of dissent and argument there.
The principle difference of Mastodon to Twitter and Facebook is that it's not a fully connected network. Instead of trying to paper over this, Mastodon highlights this feature. You have a "local" timeline and a "global" timeline. The global timelines are assembled dynamically based on what people on the instance follow. This allows instances to collectively shape their experience, and they tend to have extremely different feels.
For example, I was part of a private instance focused around hardware and crypto hackers, and I have a primary account on the original .social instnace. The .social instance has high exposure and plenty of obnoxious redditor-types bellowing their politics and wondering why they aren't getting applause, mixed among a wide array of more interesting people. I'm just as likely to get into a halting and awkward conversation with a Japanese artist as I am to end up talking about javascript critics with Brendan Eich as I am to see someone with a kekistan flag talking about their ignorance and how net neutrality is good.
The other private instance was totally different. The entire experience was great. You could make intelligent conversation about tech and expect intelligent responses. The network gradually connected to other people in the fediverse and even the global feed started to get very good. People began to use that instance as a bridge for good follows in the distributed software, hardware, and digital art scene. As that connectivity grew, so did the value of the instance. I'm very sad that the owner was unable to maintain it for a few months and the whole thing fell apart.
If your primary goal is to yell at people until they are browbeaten down? Yeah, that's a bad environment for that. But it's also a win, because this neighborhood may kick you out but I'm sure there is another one that will have you, no questions asked. If you're like most people looking for specific subjects for engagement and some control of the overall quality? It's amazing. Unprecedented, even! It takes time and effort for these instances to grow, but unlike many social networks there is actually a legitimate (i.e., not shitposting and moderation evading) reasons to have more than one account, or prefer one server over another.
While instance connectivity across the total network is very powerful, I think there is a lot to be said for requiring a great deal more inertia to have a hastily penned 200-500c comment flashed in front of millions of people. And in many ways it's the same principle as Hacker News was founded on: it's a cul de sac in the internet, a smaller place where rules are more enforcable and governance is slightly less arbitrary. Unlike Hacker News, it actually has controlled ways to knit together. Imagine if my viewing habits started to affect the top feed.
Finally, as an aside, please stop with this "echo chamber" rhetoric. It's counter-productive. There is nothing wrong or shameful about community selection. No one is obligated to engage with anyone else. We here at Hacker News are beneficiaries of this. As problematic and lacking in tradecraft as this forum often is, its still a much more reliable place to gain technical insight than many subreddits. Similarly, Stack Overflow is a "tech echochamber" but what echos is the community consensus on how to build things. That's a good echo, better than the noise you'd get on Yahoo! answers. The only people who have a vested interest in disrupting these dynamics are people for whom consensus, truth and trust are objectionable concepts.