Or another way to look at it: if you model the earth as a perfect sphere, there's a one-to-one monotonic relationship between geodesic distance and distance in

Or another way to look at it: if you model the earth as a perfect sphere, there's a one-to-one monotonic relationship between geodesic distance and distance in a straight line through the earth. Points that are equidistant by one metric are also equidistant by the other. So you could get exactly the same diagram by finding the 3D Voronoi diagram of the airports as a collection of points in space, and then taking the intersection of that with the geoid.