They tied one knot right.

That's a figure 8 on the left, and looks like not quite a double fisherman's on the right. They have created a large loop of rope, so it isn't two strands, it is one anchor. This is the tree above the X. It lost some roots when fungus attacked it a few years ago, so I don't use it by itself as an anchor

They didn't get it over the edge despite all the extra rope, and they used one locking carabiner to attach the climbing rope.