With this footage, you can clearly see how the Earth curves upwards (towards the sun) as well as downward showing that a fish eye lens was used, so the apparent curvature of Earth is simply the bending of a straight horizontal line. It is most drastically apparent near the beginning when the concave (upturned) horizon begins to level out then become convex.
If this does not make sense, check out this video explaining how fish eye lenses make straight lines curve upward and downward depending on the angle of the lens in reference to the horizontal line.
Here are some screenshots of the earth curving up towards the sun.
