When deleting a collection, you get an alert saying “Are you sure you want to delete this collection? This operation cannot be undone.” This makes it seem like notebooks within a collection might be deleted. In fact, that’s not what happens (phew!), but could update the wording to make it a little more explicit.
“Are you sure you want to delete this collection? Your notebooks will not be deleted.”
Seems to be the same situation as an album of photos on an iPhone. Removing an album notifies you that “the photos will not be deleted”.