Deut. 18.20
However, the prophet who presumes to speak in my name a word that I have not commanded him to speak or who speaks in the name of other gods, that rophet must die. 21 And in case you should say in your heart: "How shall we know the word that Jehovah has not spoken?" when the prophet speaks in the name of Jehovah and the word does not occur or come true, that is the word that Jehovah did not speak. With presumptuousness the prophet spoke it. You must not get frightened at him.
It seems clear to me that the WTS is a false prophet in anybody's book. I accept a point in a way, if a person is an atheist who believes that all religion is false then why worry if the WT identifies someone as, say Babylon the Great?
But anybody is interested if a group predicts the end of the world, the death of us all. That affects everybody and can harm peoples lives. Also, I recall the WT once saying that the role of a prophet was to speak for God, not only to fortell the future. So if they claim to speak for God and "go beyond the things written" by adding their own words - they are a false prophet.