It’s implied the vast majority of the Kaldorei survived, but let’s be clear here: the Kaldorei just lost thousands of people and the center of their power and the Alliance’s power on Kalimdor.
It was absolutely an attempted act of genocide by Sylvanas, who did it as a combination of her own malice and what the United States did during WWII: she was hoping to break the Kaldorei’s will to fight.
Unfortunately for her, she gravely underestimated the resilience and willpower of the Alliance as a whole. While she can certainly beat Anduin in matters of experience, the young king of Stormwind has proven far wiser in matters of leadership than his young age would imply.