That's a fucking stretch and a half.
There's a difference between killing people and not helping them because you're in no position to help them... Let's not pretend the Alliance was not already fighting the scourge on it's own fronts... What were they supposed to do, somehow teleport a whole army (that they couldn't spare, because all of their forces were already barely preventing the scourge from flooding south) across enemy lines all the way to Silvermoon? We all know they didn't have the capability to help.
The Alliance didn't start the war... The Horde did, obviously the Alliance is going to keep fighting until they are no longer under threat of destruction...The Alliance has also been fighting to eradicate the Horde on both Kalimdor and the EK. Both the Horde and Alliance are guilty of that.
She may have said that was the goal at the time, but we all know it wasn't, because we all know her real goal in the war now, and that was to feed souls to the Jailer.Sylvanas' reasoning for lighting the tree on fire was to break the Alliance's hope.