The islands are what's left of Atlantis.
An ancient civilization
destroyed by the sea a long time ago. It just makes sense. Ok, so it seems to be on the wrong ocean. But what kind of legend is 100% accurate?
- There's a mythic civilization of Mu supposedly located in the Pacific ocean. Depending on who you ask, it's either separate or synonymous with Atlantis.
That is, they're alternate-universe analogues of each other.
- ...perhaps suggeting that Nation and that scene in Color of Magic actually take place in the same Alternate History version of Roundworld?
- Possibly it's the version from the original Science of Discworld, that developed without the elves' assistance and left those differently-spelled signs behind at the space elevator's entry-bays. It's not impossible that the wizards' interactions with early humans in the second book gave the resulting iteration of civilization a different slate of place-names thousands of years later, while incidentally cultivating an unfortunate taste for things like sailfin-crocodile eggs and tree-climbing octopus.