Despite the fact that the world is not rounded, the horizon doesn't curve, and it's effectively flat on an infinite plane in all four directions, there's no reason for anything made of blocks to appear curvy. The moon is one giant block in the sky, and that's why it's square. Its face constrains to point straight at the earth at all times, similar to how real-earth's moon does. However, since minecraft doesn't rotate on a tilted axis and the sun and moon are on constant arcs, there can be no phases of the moon, so it's always full. So we're looking at one face of a cube in the night sky. There is no argument that it is a square moon. The only argument should be, if it's mined, will it be made of stone, or delicious cheese?