what he saidI don't see your point are you asking why the kyuubi wasn't pierced by the Kusanagi or why you don't require elemental chakra to use morning peacock?
I think the OP is asking why the Kusanagi was unable to perce the Kyuubi cloak when it had as much hype as it did yet Kakashi's lightning chain cut the other jins arms off.
I was wondering this myself, I dont know how to answer