I don’t think they forgot. They are indeed going with her having been lying, as Danuser answered at the Blizzcon Q&A:
- As for why Sylvanas would lend an arrow to aid Varian, there was a long term strategy where Sylvanas needed to get into a position where she could take over the mantle of Warchief, and she couldn’t do that if she was aggro and everyone was against her, so she had to bide her time, and now we’re seeing the fruition of that in Shadowlands.