I would say no, because it's so formal and uncolloquial. You'd use "therein" to talk about things related to documents more than the sleeping habits of animals. So it might be correct, but "in there" would be better.
I thought this sentence was using the separable verb "inslapen", to fall asleep, but I see from my red X that I was wrong. How would one say that the rhino is falling asleep? Would that just be "De neushoorn slaapt in" ?
When I did Swedish the iPad would give me % fluency and the Tablet didn't, so I certainly have experienced differences moving from one device to another. Perhaps there are more things in heaven and Earth than are dreamt of in your understanding of the app. :-)