I suppose this means that some Immortals may actually be in the Mortal Class or Hero Class in Lords of Olympus terms. And a quick look at some Immortals' stats actually confirms that. So it's a matter of coming to an arbitrary definition of those two classes, which effectively establishes the rest of the ratings as Olympian Class or better.
Speaking of Olympians
They're not in Mystara. As per a Vaults of Pandius article, the Olympian pantheon does not exist (visibly) in the Mystaran universe. It does make me want to research the Milennian culture and the Immortals they worshipped, though.
But this does open the gateway to playing -- in the vein of Lords of Olympus -- true children of the gods. If the Paths of Immortality are therefore ways of welcoming people who might secretly have the blood of the gods in their veins, or transform them into god-like beings through mastery of the magics.