They are wide angle lenses, all lenses have it distortion, but fish eye doesn't increase with distance and warps less, the closer an object is to the center of the frame. At very high altitude where the curve of the earth would be visible if the earth was flat, when the horizon passes below the center of the lense it would Bend upwards equal to the amount it bends downwards in the same position opposite the center line of the image. If there is a discrepancy, there is curvature. So FL can still be debunked.