I just don't see how to explain the curve in this feed can be real and the earth can be flat at the same time.
Unfortunately imagery like that doesn't tell us much. For example, a wide angle lens can make a ruler look curved, just due to the way the lenses work. You might want to check this out: https://en.wikipedia.org/wiki/Wide-angle_lens
Example of wide angle lens adding curve to 'straight lines':
https://www.digitalphotomentor.com/photography/2015/05/wide-angle-lenses-750px-01.jpg
Lens distortion is a very real thing. That's a wide-angle camera, and the result of that is that everything appears to curve around the central point of the frame. Things at the top seem to curve down, and things at the bottom seem to curve up.
Actually, there's plenty we can do to deal with lens distortion of you put your mind to it. If you were serious about using a distorted image like this one to figure out the straightness or curvature of some feature, you could use your knowledge of lens distortion to undo it. Real scientists don't just throw their hands up and say, "I guess we'll never know." As we've said, objects at the top of the image are bent down (like the horizon in that image). Objects at the bottom are bent upward by the same amount. Objects in the center of the frame are not bent at all. So the first thing to do when you see a distorted image like this is check to see if the interesting part of the image ever passes through the center of the frame. If you can catch your object center-frame, just take your measurements there. If not (as in this case), you'd have to calculate how much curvature there is and account for that. The math for it isn't all that hard, but here's a link to how you can do it using software:
https://photography.tutsplus.com/tutorials/how-to-remove-lens-distortion-from-drone-video--cms-28958And if you were to do that with this image, every knowledgeable RE will tell you, you should get a very straight line for the horizon at that low altitude. That's what RE says you should get.
Expecting to see the curve is the FE mistake.
What I really want to impress upon you is that there is more to learn if you dig a little deeper. There are tiny clues everywhere if you are willing to look for them, and by piecing them together, you can discover the full picture.