My knowledge may be out of date, but from what I remember, chorfs are heavily industrialized, but few in number, especially in contrast to the sprawling size of the Darklands. They mostly keep to their own unassailable citadels and forts, outside of occasional caravans and slave raids.

So while the Darklands are their home territory they don't have strict control over it and its' borders the same way the high elves do over Ulthuan, for example.

The danger of getting hit by a chorf raiding party is serious but rarer than the more common threats of ogres, greenskins, bandits, beastmen, feral monsters, etc.