The zip lines go ACROSS a valley. Doesn’t take much of a drop in elevation to make it work. It probably take just a very short hike up the other side of the valley to make the return trip. A short hike up a valley is pretty easy for the return trip.
These kids are lucky...most of us would have to enlist somewhere to do this kind of stuff.
That explains how you could get back, but it does not solve the problem of how you get the pulleys that you ride down back to the original side. Also, the photo shows a pretty wicked looking creek down at the bottom.
I think the pulley system has to have a way to pull yourself back up the line when you go home. If I was writing the story, I would have found out and explained it as it is the most obvious question. The idiot reporters of the world have no curiosity anymore.