>There is clearly something I don't understand here. Consider an int -- you can dynamically allocate one, you can put it in a tree putting a box into a tree will still require other data applicable to the tree itself, same as an int), and so on. So I don't understand the point being made here.

To make a tree, you need something allowing an object (a tree node) to own an object of the same type as itself (another tree node). There are various options for this: raw pointer, unique_ptr, shared_ptr, auto_ptr, box. Without using one of those options, the tree isn't possible. That's the author's point. std::box can be used in place of one of the other pointer types.