Not neccessarily. Spirits are obviously not real (or if they are, the way we portray them could never be how they truly exist) and therefore, each story gets to make up its own rules about them. However, humans do exist, and severing tendons and that kind of crap would actually probably not allow Cloud to use that arm for the rest of the battle, if not his life. That's a rule in the real world that must be followed. With spirits, you can do whatever you want. You can make them physical or metaphysical entities. So no. Being able to shoot spirits (having their own rules) in one story, does not dictate Cloud being able to move his arm (having to follow laws of the universe) in a completely different story. Your point is imprecise, incorrect, and irrelevant.