Chaosmancer
Legend
Slings are plenty deadly in 5e.
A PC that has managed to narrowly dodge and avoid the worst blows of a Greatsword repeatedly can suddenly be taken out with a single hit from a sling.
Or in other words, making conclusions about the physics of a D&D world on the basis of its combat system is beyond ludicrous.
Of course they are deadly. You realize you can kill someone with a stick right? Sticks are deadly as in they can cause death.
But, is a stick deadlier than a 50 caliber bullet fired from an anti-tank gun? No. So, both are deadly, one is deadlier than the other. And, DnD has created a tier system for which weapons are "deadlier" in DnDland. Which is where gnolls and halflings are.