The Genesis and all the other lore of the game never mention anything related to the idea of planets. Lore-wise, I think we can assume this concept simply does not exist in Tibia. Other astronomical bodies, such as the Sun and the Moon, and frequently mentioned, but not the planet concept. We don't even know how big it is, or if it's even round (haha). On The Birth of the Elements (Genesis Charper III), it says:
So it came to pass that Tibia, the living core of all creation, was born. It derived from the element of the earth, whereas Sula, the mighty sea lapping gently against Tibia's shores, was created from the element of water. Air rose over creation and spread itself like a protective blanket over it, while fire was to be the fundament, warming the land with its eternal flames.
So the Genesis does call Tibia the first idea of the world we live in, since before it all we had was void and the gods. In a way, it could be considered the name of our planet, since it was also where the first creatures and humans were created. Surely, as mankind grew and explored the sea, they discovered and inhabited islands, some almost as big as the main continent, and gave them their own names. You can also look from skeptical point of view and consider even the Genesis was written by a human, meaning all of it is based on at least one person's interpretation of history and may not be exactly true.
From the game point of view, I guess they never thought about it. It took quite some time between the creating of the game and the creation of the first islands, and even more for big cities in other islands, such as Edron. Before that, the main continent was simply expanded as if it was being discovered.
I don't think we can call servers "countries", since all servers are identical (at least in terms of Geography). I'd say they are closer to being dimensions - the same structural world, but with different events and different inhabitants. We even have methods to travel between them!