No, rather italic of me to assume. (Harhar)Rather bold to assume that based on that item, you're not going to accidentally lose that Greninja. If the Greninja does get cloned, it's because it's the only Pokemon you have with you. Flavourly speaking, it doesn't feel like the Greninja is being 'transferred over' if this is the case.
We could also assume that that's how those Z items work; they're like infinite TMs except they are attachable. I seriously do not believe they made the item infinite just for the demo so you couldn't lose it. Especially when the only way to lose it as you said is with the catching challenge which is after the main demo event where the item was suppose to be showcased.
Anyways, that basic joke aside, in a full game or under different circumstances I would agree with you.
However, based on what I have seen in demo game play alone (the leaks didn't help confirm or deny anything), I feel like my assumption is an accurate one.
Feel free to disagree if you want, and I will make sure to post here to let everyone know I was wrong if I turn out to be.