Synecrtism is always weird in settings where the gods are real.
Yeah, though maybe history was different on Earth 16 since they existed, so the whole King Poseidon thing never happened and Zeus was always the head honcho.
The gods in this story don't seem to be as malleable as the gods from the Vertigo comics, so they may not have changed due to the belief of the people changing over time.
But I may be wrong.
Historically speaking he didn't even have that title originally, with Poseidon holding the title before a massive restructuring of the pantheon sometime before The Bronze Age IIRC. That same restructuring is what got us things like Dionysus being a wine god instead of whatever eldritch abomination we're pretty sure the Greeks used to see him as, even if he kept madness as one of his domains.
Edit:It was actually during The Bronze Age that this was true, here's a video by Overly Sarcasitc Productions that talks about the subject.
Ahh, Dio, you truly are a wild one.
That scene where the king looks at him and the horns show up is low-key terrifying.