There's plenty of text sources for this information, a model doesn't have to see anything. We have sea/land masks in GeoJSON files etc.
Going with the hypothesis that their "geographic knowledge" simply reflects coordinate density in text (more populated areas → more mentions → better land prediction). If true, plotting all Wikipedia coordinates should correlate with the "LLM world map" here.
This is exactly what you see here, if plotting all GCS co-ordinates extracted from Wikipedia: https://github.com/Magnushhoie/Wikipedia_Coordinates_Visuali...
This implies that larger models aren't necessarily "smarter" at geography - they just have bigger memorized datasets.