Flat earthers are reality-deniers. On the one hand they claim that the camera lens creates the illusion of curvature on the horizon. On the other hand, they are too blinded by their agenda to see that there IS a horizon!
If the earth were flat, one would see endless land, and neither the eye nor the camera would see far enough to detect a horizon at all. If the earth is round, then the horizon is about 55 miles out, as the man in the video claims, and one sees nothing but sky beyond that.