For my understanding, the joining in the seam's definition means that both edges are put together, like touching each other, or very close to each other. And in this case both edges (metal) are kept separated by the black stuff, which by the way we are not sure if it is rubber or other type of material.
It is not a line, it is a space filled with the black material.
It's not made out of wood nor vulnerable. Vulnerable to what?
What did the Kangaroo do to deserve that? Post in MacRumors?