And that can be seen in the new cinematic.
During Warcraft 3, the “Mighty Lich King” calls Arthas to defend him, because he’s helpless against Illidan and the Naga and that means that the Lich King has no physical power back then.
Then, when Arthas puts the helm on, the scourge massive mind-controlling power, fuses with the undead physical prowess(through Frostmourne) and this is the moment that the Lich King is at his strongest state.
When we defeat him in WotlK, the moment Frostmourne (his physical power) is broken, he becomes helpless again and he is vulnerable, and that’s the moment we kill him.
That just means that Bolvar without the power of Frostmourne (which was taken by Frost Death Knights and later used to weaken the sword of Sargeras) is just a very powerful mind-controlling entity, that kept the scourge at bay, but its physical power is next to nothing.
So if you take a Sylvanas that has made deals with very strong entities and takes an enemy -which has no physical power - directly, this outcome is not that strange like some claim.
Please… We all love the Lich King. But see things as they are. And stop your exaggerated worshiping of the Lich King.