I'm guessing they mean that given a bunch of correlated nodes but no collider (in which case the casual graph must be a tree of some sort) you not only don't kn

I'm guessing they mean that given a bunch of correlated nodes but no collider (in which case the casual graph must be a tree of some sort) you not only don't know if the tree be bushy or linear, you don't even know which node may be the root. (bushy trees, of which there are very many compared with linear ones, would be an instance of Gwern's model* of confounds being [much] more common than causality?) * https://news.ycombinator.com/item?id=41291636 https://news.ycombinator.com/item?id=41291636