The traffic itself is not the interface. The interface is, in one sense, represented by the rules and conventions of how one system talks to another, and in a different (but related) sense, it is a piece of code on the periphery of a system that implements those rules, making the communication possible.
When two systems communicate, data can flow in either direction, depending on what exactly these systems do. For example, the internal code of some system may make calls to its northbound interface layer in order to talk to a higher level system; the northbound interface will then communicate with the southbound interface of the other system, which will in turn interact with the internals of that higher level system.
So it's more like this:
The interfaces define the way the communication happens, making the internals of each system decoupled from each other.