Ok, I never thought this has to be made clear. I thought something as obvious as what burning of Teldrassil and wiping vast majority of kaldorei population is will never have to be talked about, because to someone with at least 2 digit IQ it is as obvious as where the sun goes during the night.
However after seeing certain comments in threads I realized that horde players are actually even dumber than I knew.
So let me explain to you, just as if you were sitting in school- Genocide is intentional mass killing of one specific (in this case) racial group with the intent to wipe them out- partially or as a whole.
(This is something that’s objectively true & basic stone on which I’ll explain the following.)
What happened at Darkshore before Sylvanas gave the order-
She had conversation with Delaryn, where she tried to make her feel bad about the weigh of her failing in an attempt to bring down and kill her remaining hope. Since Delaryn showed her tenacity towards such an attempt, implying she believes that hope cannot be killed, it enraged Sylvanas, putting her into a blind affect.
Ever since battle for Quel’Thalas, she felt lost and hopeless for considering herself a failure to her people. For not being strong enough to defend off the gates to The Sunwell and allowing Arthas to defile it with necromancy, ultimately destroying it. It is a personal injustice Sylvanas holds to her heart, and because she can’t let go of the fact, that she wasn’t able to stop Arthas and the scourge from murdering majority of the Queldorei population, she now wanted to inflict the exact same pain to others, as a way of coping with these emotions.
And That’s the main point - She wanted to make Delaryn feel exactly as she felt when she fell to Arthas. To watch all of her people die right before her eyes, to know that none may survive. To live forever with the knowing of failing those she was trying to protect
That’s why she gave the order, which she thought on the spot, invoked through an impulse in her blind rage, which makes this act a genocide- Intentional wiping of one race.