This! Exactly.
Though, I don’t recall her using her Banshee form in WoW until BfA, they did show it, at least in the intro cinematic. However, to me, what was more indicative of her strength was her easily deflecting Saurfang’s first attack.
It violates the principle of Chekov’s Gun. It’s a metaphor that states, “If a gun is placed in the first act of a story, it must be shot by the third.” However, the consequence of that is, “If a gun is shot in the third act of the story, it must be placed in the first.”
They didn’t show us anything of Sylvanas getting power. It didn’t feel like a reveal, because there had been no (or minimal) foreshadowing to prepare our brain to interpret it as a “reveal”. Instead of, “Oh, those are the powers we’ve been hearing about”, we all thought, “why is a dagger blocking a war axe?” Because we had no other information with which to contextualize the fight.