Think about it this way.
Let’s say I have a magic laser gun that will burn anything I point it at from here to infinity. I can shoot it once. I know Princess Fizzbang is hiding somewhere in the forest, but I can’t see her. So if I shoot the laser gun at the forest, odds are I’ll burn a hole through the forest but not Princess Fizzbang.
Now imagine, instead, I summon a meteor to land in the center of the forest, engulfing it in flames and burning the entire thing to the ground. Somewhere in the ashes I will find the remains of Princess Fizzbang. You can hide from fire all you like, it’s going to roast you.
That’s Stealthy. It makes it impossible to target an enemy, but if you are firing off a wide-spread “everything in this general direction” blast you’re going to hit the hidden enemy that’s still there.
(This sort of works that way in D&D, too. If a spell requires “line of sight” I can’t hit a stealthed or invisible enemy. But if a spell hits “a cone” or “a sphere” and the enemy is within that area, they are going to be hit.)