A coast can be steep without being an edge
Floating shelves meet the ocean at ice fronts; bedrock and ice also form inland mountains and plateaus. The visible height above water differs from the submerged thickness of a floating shelf. A close coastal image does not establish the geometry of the whole continent.
The treaty covers a geographic region
The Antarctic Treaty applies south of 60° south latitude, including ice shelves, while preserving specified high-seas rights. It provides for peaceful use and scientific cooperation; it does not define a barrier around a flat world. An actual trip must follow the applicable national permit, environmental and operator requirements.
Compare polar predictions
Record solar directions and day length with latitude, longitude and time. Compare routes between southern locations using the same measured baselines. A model that stretches the southern world must account for these distances and simultaneous sky directions together.