I guess we just have different definitions of open and closed areas. Khoronis, to me, was never closed off. It was only difficult to enter, fenced off so to speak. But it doesn't require the player to solve a certain quest or be a certain level, have a certain skill or piece of equipment, be at a certain point in the story or point in time or even behave in a certain way. You can get a permit, you can buy farmer's clothes, you can climb the wall, etc. You can always get in somehow, it's never scripted (as you call it, artificially) closed.
But I do agree that the end game cave was closed off..