From my understanding,which is limited when it comes to Mandalorians and their history, but from info others have given me,Mandalore is a title but before it was a title there was a Person who's name was Mandalore and this person helped in the Exar Kun war but was "killed" on Dxun by some of the animals on the moon.
The way the question was asked and answered it sounds as if the character in Sith Lords is Suppose to be the one who was "killed" on Dxun.
While the person killed at the hands of Revan was one who simply held the title of Mandalore.
So the real question would seem to be,how did this character that most have thought to be dead,with no way to espape or survive in such a harsh envrionment do so? The war with Exar Kun is 40 some years before Knights and Mandalore wasn't young as it was I guess.
It's something that Obsidian needs to have a really good answer for because EU purists are going to have fit if they don't.