its just the opitcal law of linear perspective that we see everyday.

As I've tried my hardest to explain, it's not something we see every day outside of the sunrise and the sunset.

Do me a favor, and try what you asked me to do. Lay your head down against the ground, and have a friend walk away from you. You'll see them get smaller and smaller, but never to the point where they dip below the horizon before fading away from your field of vision.

if that video is real

That's a great statement as well. We don't know if it's real. We have no details about it, and it's also significantly out of focus, so hard to make out exactly what we're seeing.