You can determine the relative position between the ground you're standing on and an object in the sky by looking at it. That's enough to invalidate models that are clearly wrong about the relative position of the sun at sunset, such as this one.

It may be possible to create a flat-earth model of how the sun works that explains time zones and seasons while also allowing for sunrises and sunsets. If it's not, then that's the other premise we need to start inferring facts about the shape of the Earth from looking at the sun. I don't really care to try that now, I'm just dunking on a particularly ridiculous model.