What reference has been given? Where has Bolvar’s power been established anywhere in the lore? If the question were “Who would win: Paladin Bolvar or Death Knight Arthas” hands down it would be Arthas, he was in a different league. So give them both the power of the Lich King and Arthas is still much more powerful than Bolvar. Arthas wasn’t immune to cheap tricks either, he had to retreat after being hit with the Forsaken Blight.
No real evidence has been put forth to prove that Bolvar was exceptionally strong, and a few dragonkin from Vanilla isn’t going to fly. Bolvar is also still alive, he might gear up, get pissed, and beat Sylvanas like a drum at the end of Shadowlands. I doubt he is just going to be the catalyst by which the expansion is introduced.
If we spend a few patches helping Bolvar get his groove back, I would be more than happy to see him end Sylvanas. Until that happens, he was rightfully outmatched.
He was using a two handed hammer, iconically the slowest but hardest-hitting weapon in RPG history against a very fast target with evasion capabilities.