To be honest I disagree.
I think that the fight cinematic could have given the opportunity for more back and forth, but in the end Sylvanas is working with the bigger player.
The Lich King wasn’t a force from the Shadowlands, he was an anomaly created by the Legion. Unless they retcon the Helm of Domination to be an artifact from the Shadowlands then it’s just an object that someone made. A powerful object, but just a regular object with special powers made by people in the living world. It can tap into necromancy but that’s it (powerfully, though).
We know that Sylvanas is working with an ancient force from the realm of death. This isn’t the territory of mere liches, knights, or mortals, anymore. This is some deep old magic that goes way beyond necromancy, way beyond undeath, and way beyond the capacity of any living being to date.
I can’t imagine the Lich King would have stood up to the Jailer himself and that’s who’s backing Sylvanas. If she needed more power she’d have had it available. If I were sending an agent to take out the Lich King you’re darn tootin’ that they’d have full access to “death” to just wipe out that Lich King without a fight at all.
I daresay the fight’s existence was more for our benefit than anything else. A way of showing she’s become more powerful without making it look silly. Sylvanas absolutely could have death-blasted Bolvar and been done with it, and she absolutely could have done it to Arthas as well.
They both might be undead and immune to ‘dying’ in the traditional sense but when death comes for your soul it doesn’t care if your body is alive or not.