It's a subculture. I think the key is in the definition of 'everyone'; I doubt anybody imagines everybody in the world switching to Gemini from the web. So 'everyone' in this context means all the people who have collectively agreed to stick to minimal text format.

You could try the same thing with a community dedicated to minimal HTML--heck, you could say that's what HN itself is, almost--but it would have no effective border. You'd keep falling back into the bad old web. This makes the border explicit, instead.