And for the completion of the argument, there are many historians and students of the law who DO consider Hiroshima and Nagasaki genocide.
And no. Not all acts of war are genocide. If, for example, civilians are killed as part of combat actions. Which wasn't the case in Teldrassil: No combatants in the city, of either side, bar wounded. If you aim to destroy enemy war machine or infrastructure, specifically, and kill civilians in the process. But Sylvanas doesn't do that. She destroys not only the traninig grounds, barracks, armory, etc, nor does she target them and accepts collateral damage. Can you name a single instance where members of the alliance killed members of the Horde for no other reason then them being members of the Horde, on orders of their superiors? Without the goal to capture territory, resources or information? Or to defend territories, resources or information? No. But here, we have the Warchief of the Horde ordering the destruction of Teldrassil with all inhabitants, for no other reason than to destroy it.
She aims to destroy it all. Root and stem, literally. Along with its people. She accepts their death as a consequence of her action. She accepts that there is no way for them to escape. She could have warned them. Could have given them time to evacuate. Could have taken them prisoners. But she didn't do any of that.
She killed them. Ruthlessly, indifferently. And that is genocide.