That is patently untrue. To my knowledge, every flat earth believer accepts the video evidence of high-altitude balloons. Nor do we anticipate some curvature should be evident at height.
Even the most feeble-minded among you must see that the video is filmed through a wide-angle lens. The horizon appears amazingly curved, or flat depending on it's orientation in the video. At one point it even looks concave.
This thread has been rehashed several times over.