Earlier in the Light Prison, a calamity stood inside the empty room after closing the portal connecting it to another dimension. The calamity pondered his recent loss as it stood, waiting for his enemy.
Cassius was the supreme being.
This belief was absolute.
It never wavered in the Battle of Gods and was not about to be broken now. As he stood inside the prison cell where Zonas Mantra used to live, Cassius thought about this truth. He lost against Morana, the Gatekeeper, not Arthur Silvera.
Thus, he believed that Cassius was still the supreme being in this world. It was a different matter if Gaia's sister interfered. Cassius has failed to evolve into an astral being because the gatekeeper was the final obstacle.