Parable: a usually short fictitious story that illustrates a moral attitude or a religious principle.
Prophecy: the inspired declaration of divine will and purpose.
Illustration: a picture or diagram that helps make something clear.
So the WTB&T$ throughout these articles called these stories alternately illustrations and parables (which are two completely different things) and applied them as if they were prophecy.
As for sheep and goats, if it is an illustration, then it is simply a word picture to make clear that to be a sheep one should show the qualities of those who go to Christ's right hand. It cannot, by the WT own words, be a prophecy as they stated the sheep and goats were not literal. And it also cannot be a parable as then the story would be fictitious. (Of course we all know it is)