> can others just subtly keep way more items in their working memory at once?
This is how my mind works, but I've actually found it to be extremely unusual, and I would say your estimate of 90% of people's not thinking like me is, if anything, low.
I normally have anywhere from 4-7 things going on in my mind at once. This is comfortable for me. It takes concerted effort to keep things from popping into the 4th slot if it ever empties out.
I start to notice overwhelm at 10, and become incapacitated at 12. Whenever I reach the point of needing to brain dump and write things down because I'm getting overwhelmed, there end up being 10 things on the list. At 12, I short circuit.
But I'm ruminating on 4-7 things at any given time. My dreams are mostly lucid, and this happens in them too.
I don't know how to answer the "how" question, because as far as I can tell, I've been this way since very early childhood. I remember being 3 or 4 years old and having an adult tell me it's not possible to talk about one thing when you're thinking about something else, and laughing openly because I thought they were kidding. Of course it's possible. What other way is there?
But as you noted, I've found my experience to be extremely uncommon. I think most people can handle two or three things if they have to, but that's all. I can no more understand what it's like to be able to do that without being understimulated than I imagine they can understand what it would be like to experience my brain without being overwhelmed.