She didn't threaten to destroy it, she threatened to withdraw her support of the Forsaken - a world of difference. To quote WowPedia:
With the Horde now dedicated to a Northrend campaign, Lady Sylvanas and a group of Royal Dreadguards traveled to Sunfury Spire to gain the support of the blood elves towards the war effort. She was met by her old comrade, Lor'themar Theron, Regent Lord of Quel'Thalas, along with Grand Magister Rommath and the current Ranger-general of Silvermoon, Halduron Brightwing. Sylvanas instructed Lor'themar to send a force of blood elves -- magisters, Farstriders and Blood Knights—to Northrend, though Lor'themar was apprehensive, reasoning that he had only just overcome the war on Quel'Danas. Sylvanas noted that the blood elves are a part of the Horde only by her hand and that if they do not mobilize they would surely lose its support - and Forsaken support in their lands. With his arm twisted, Lor'themar relented, and agreed to send a contingent of blood elves to the Undercity.
If you can't make a point without using hyperbole and outright lying then you had no point to begin with.