I think he means that the code that doesn't do anything, just describes the interface between two software components is not a code (that you should be prevented from copying) because of its one to one equivalence with the interface.

Describing interface with words is exactly the same as declaring it with code. So such code is nothing more or less than verbal description of the interface (or any other description of it using any other syntax).

Not sure if that helps. If you can copyright interface, or just algorithms. Unless Java interface was specifically made free and open by Sun then it's really clear cut case.