They're all people that I know, and 98% of them are mutuals. I regularly go through my list of accounts I'm following every 5-6 months, and get rid of people who have disconnected from me, have gone inactive, or I don't really know.

I think the part you're missing here is that there are certain parts of twitter where the density of interconnection is really high, so you'll know people because you see them in the comments of a lot of threads, or through retweets.

It's really not that hard to end up knowing 1,000+ people if you engage with a group or "corner" of twitter. Even moreso if it's multiple corners. (Like, AI, but also just frens, but also a little political corner, people from specific cities, etc.)