Earlier quoted context omitted.
The big tent for everyone sites are toxic because people don't interact as closely in real life with people from different groups. We group for a reason. Within those groups common understanding on certain ideas is the connection. With that connection you let down your guard, you can be this side of yourself freely. In a big crowd you have to wear all faces to relate to everyone.
Anecdotally, I've seen far more polemic and vitriolic posts on so-called alt-tech social media sites compared to even Twitter and Facebook, which are horrendous in this regard. Sites like Gab, Voat, Ruqqus, etc seem to be filled with the absolute most toxic, angry, and hateful discourse imaginable. I've noticed sites that tend to be for something other than "just chatting" tend to do better: many hobby specific forum…
Furthermore, if someone is just there to be toxic, you have an easy justification to shut them down: "I'm sorry, but this is an underwater basketweaving Discord, please stop spamming #offtopic with why you think we should call in the National Guard to fight the war on Christmas." Many small communities have very strict regulation of political speech purely because it causes most of their problems.
Alt-tech is trying to collect all of the toxic bullshit in one place, so obviously they're going to be way more toxic despite their small size. It's also possible for niche communities to have their own toxicity problems, too. It's more that large communities are just inherently more difficult to govern.