X-15 at 317,000 feet proves Earth is Flat?

Flat Earthers have claimed that the X-15 footage from 317,000 feet proves the Earth is flat.

For example, in this video (now deleted):


Or this other Flat Earth video:


Here is a backup video with the original footage:


I say they either purposely or ignorantly used a frame where the infrared camera has the horizon washed out because a few seconds later in the SAME VIDEO you can see the horizon very distinctly and it's very clearly curved -- and more than that, curved exactly as much as we would expect.

Here is the frame they picked:

But just a few seconds later in the video we can see the horizon much more clearly:

More over, we can see from the dimensions of the aircraft that this is a fairly narrow Field of View, I estimate about 30 degrees here:

In my image the horizon view is about 1330 pixels wide, plugging that and 317,000 feet with 30° FOV into my Horizon calculator we get an estimated 15 pixels for the *apparent* sagitta height.  Which matches what we see in the footage.

Flat Earth fails again.