If you consider the novel "Rise and Fall of a Dragon King" canon, a real Athasian dragon would be even more powerful than the one listed in the Darksun MM.
According to Hamanu, the protagonist of the novel, one was supposed to undergo a ritual involving the Dark Lens and several spells known only to Rajaat, before starting the transformation.
This ritual was only ever performed on Hamanu, so Borys and the other kings were not true dragons, only dragon hybrids.

Then again Dragon Kings are notorious liars, so who is to say if Hamanu was telling the truth.