Both. As it states on the page you referenced:
The Action instance is always pushed onto the value stack. Because the Action is on the stack, and the stack is the OGNL root, references to Action properties can omit the # marker. But, to access other objects in the ActionContext, we must use the # notation so OGNL knows not to look in the root object, but for some other object in the ActionContext.
The diagram you refer to is not an object diagram, but a diagram reflecting how the ActionContext is referenced in OGNL.