Because The Queen of Edgelords pulled out demi-god level powers out of her behind and rekt someone who should have wiped the floor with her while having his eyes closed.
The writing team seems to be doing everything they can to stretch the logic and circumstances in her favor and make it LOOK like she had a plan all along.
And while it did look badass and the cinematic was terrifically made …it was still dumb as hell. Sylvanas can’t just keep breaking and destroying everything she sets her eyes on and keep getting away with it with no consequences for her.