Ephrim doesn't expect it to make much sense upon the first pass. He realized that the initial explanation didn't involve any gods and didn't want that part to come off as tangential. The image that Rosinante has is inaccurate, but close enough that Ephrim wouldn't be too bothered about any confusion on Rosinante's part. It's not important, anyway.

Ephrim wouldn't mind Rosinante not believing much in gods. He's met them, of course, but pedantry about believing in the tangible versus the imaginary aside, they were as prone to despair and bad decisions as anyone else. A lack of certainty isn't a particularly god-like quality, he thinks. What was the point of having the power to create and destroy reality when they were too caught up in their doubts to save their own creations? Cruelty is one thing, complacency verging on self-destruction is another. No. Ephrim doesn't believe in gods, either.

Ephrim is back to looking at the stars again. "We were in a jar, then. The jar kept the nothing out, and that which was reality in. There was an edge to the oceans, of course, but we were discouraged from trying to find it." Simple as that. (Not really, but expanding the metaphor is pretty worthless right now.) "I am also hesitant to believe that our universes were one in the same, but perhaps your oceans came from a time when Severea was strong."