I have to disagree with that last part. Beings killed and raised by Frostmourn have their souls and their wills taken from them. The Red Dragonflight’s fire did purge the plague, which was designed to kill both living and undead. Neither Arthas nor the Plague reanimated bolvar because that Plague only kills and Arthas would not need to break Bolvar if he had raised him and taken his soul.
Bolvar was already dead or dying. There’s alot a debate around the last part, but the concensus is that it’s the power of the Red flight that animated him and Wowpedia agrees.