That is interesting. I don't have the strategy guide (yet, anyway) so I didn't know that. I have read the novel, though, so I assumed there were other continents. Does it say in the strategy guide how many Cocytans survived the ice cap melt? And how they got to the top of the highest mountain? I guess a solar flare with enough power to do anything would melt them quickly (but I'm not an astronomer).
I was under the impression that they constructed the islands as a welcoming star port for aliens. I think the creator says something about that in the game, and that is definately echoed in the novel. It suggests to me that there are other continents or at least other islands.
That leads me to ask: what should be considered as Dig 'canon'? The game, the strategy guide or the novel? I place a vote for the game.