It's a practice thing. I know three folks who can hit a 1" target, every time, with a sling, at anything from 20 to 80 feet.
It's just having good hand eye co-ordination, and lots of practice.
One's an archaeologist who had watched a laddie in Portugal, a shepherd out on the hills all day long, and thought it such a perfect example of a simple tool that survived the test of time. So, he had a go at it, and he got very good at it.
Incidentally, that shepherd boy had a hand gun tucked into the waistband of his trousers, but ammunition is expensive, while wee stones are literally just there for the picking up.
I suspect that the art survives mostly among those with few resources to waste, and enthusiasts.
One of the others is my brother. He always was accurate, with anything, and it's the same with a sling.
He says a blowpipe (which is illegal in the UK now) is a really accurate tool, especially for birds. He pinched an entire bag of my beads years ago to make up darts for his. The beads were the perfect size apparently