That image is distorted due to the bulge of the aircraft's window. The fish eye effect does not necessarily affect all areas of the image because this particular window does not bulge outwards uniformly. It's most likely a large vertical dome bulge which runs down the middle of the window.
As for curvature as seen from the edge of space, it comes from the fact that on a flat earth we are looking down at a circle. The Ice Wall and other continents of the earth are still tens of thousands of miles away horizontally from the observer at an altitude of 100 miles (edge of space), and thus beyond the resolution of the human eye and merged with the line of the horizon, indiscernible. This is why the view is limited to the immediate vicinity below the observer.
We can confirm that we are looking down at the circle of the earth by noting that shots from amature high altitude balloons show an elliptical horizon. If the earth were a globe, all curvature seen in photographs would appear as an arc of a circle.