That was suppose to be why they have Azshara to sustain them which was the initial point of the night elves giving them Azshara at the end of SoO. Then they also have places like Suramar, Highmountain and Zandalar if they are in need of even more. The fact that they need to continue their invasion of Ashenvale is the problem, they aren't attempting to repay the night elves, if they were they would have been thanking them for Azshara, instead they are continuing the war against the night elves that Sylvanas started. The writing here is just extremely badly done and it's just showing the horde has not changed even under the leadership of the new council.