Hi Lasse, I guess you couldn’t leave the flat earth topic after all :-). As others have pointed out in comments the footage is taken with a lens that has barrel distortion “fish eye” lens. You will have to look at places where the horizon is centre frame for it to be without distortion.
Another way of capturing the curvedness of the horizon is to use a rectilinear lens (no distortion). Using such a lens it is even possible to capture the curvedness from ground on Earth. Here is a link to my findings using a close to rectilinear lens, https://flic.kr/p/WoaBqu. The amount of curve is not more than 3-4 pixels but when scaling the centre part the curve is visible.