tbh i have no idea how huge the island of Le Caillou is. Joyce calls it "a pebble", but she also claims that the entire island is able to sustain 200 million people, and Revachol is a metropolis with an "80 km radius" (that's slightly larger than LA, even if we assume that radius doesn't apply equally in all directions).

But yes, it seems unlikely that something that large wasn't settled permanently for thousands of years when it's part of an island continent with several ancient seafaring cultures.