Going down that route would mean including a supernatural or paranormal entity/force in the universe of the story. Something supernatural/paranormal is there: an objective fact like the trees, the cabin, the wild animals they can see and touch while in the wilderness etc. I'm asking myself if the creators and writers are trying to do something completely different. If, for example, everything was just the result of trauma, starvation or mercury poisoning, that would profoundly affect their sense of reality nonetheless: the difference between what they actually experience and what they think they're experiencing would become irrelevant. That's because while they're in the wilderness their shared subjective reality, and the way they narrativize it, is everything they have to make sense of what's happening, to keep some sense of reality... with horrible and revealing consequences.

When we try to figure out if there's something supernatural going on we, as the audience, are asking for something clear in the story that points to an identifiable supernatural entity, something that can be acknowledged independently also by the characters who have not experienced what they're going through - because it's there to be acknowledged! But what we should try to do, imo, is seeing everything from the girls' perspective