That's a possible theory if you look at Wano. At the end of the arc, Robin is shown what is an older version of Wano from years ago until the water rose too high. Its possible that years prior in the time of the unknown that the world was much like ours in several large masses of land surrounded by water and that all of the islands are technically part of those land masses.
Maybe it has to do with the weapons and One Piece being a terra forming weapon that manipulated the planet to make what it was today. Although it wouldn't explain why Roger and the others were laughing. Its also possible that the One Piece isn't that weapon and that the other weapons are and that One Piece is something all completely different. We do know that one weapon can manipulate the Neptunians and potentially destroy the world.
They laugh because using the One Piece will change the world as they know it. Terraform the planet and the big sea will be gone, which will fuck up things for both the pirates and marines.
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