If there was someone that told them about the Seven Seas, it would be Averill, here is why…
Captain Averill was off on a journey to sail to the edge of the earth around this time, and discovered many lands and kingdoms on the opposite side of the world. He eventually sails all the way around the world, and ends up on the eastern shore of Magius.
Because of it saying, “opposite side of the world” Averill knew of the existence of the War Seas. We know this because Vetex states very clearly that the War Seas were on the opposite side of the world.
This could only mean one thing,
The New Greeks got to Magius by following Averill, meaning the War Seas somewhat knew about the Seven Seas. This most likely means that while the Seven Seas may not have known of the War Seas, the people of Magius definitely do. Word must have slipped out among the New Greeks to the other cultures that the War Seas existed. Captain Averill may or may not have told them about the existence of the Seven Seas, and about how a mysterious Peacekeeper defeated the most malevolent creature to ever live. At this time in the story, one month after, Averill must have not discovered the War Seas yet, meaning that Vetex was right. My prediction in the story is that word will spread from Averill when he comes, and the War Seas will finally know of the Seven Seas.