Originally Posted by
the COMET
The similarity between the names is just a coincidence. In the real world when an author created Poseidonis of course he was naming it after the greek god's name. But in DC continuity the city-states of Atlantis where created 1 million years before the passing of the Godwave through Earth, event that gave birth to the Greek Pantheon.
As for being the capital, i consider the City of the Golden Gate as so, said to be the first and largest city-state, at least in ancient Atlantis time. Poseidonis is just the one that remained intact