The theme isn't explicit. We want to add it to the narrative in game at some point, but the gist is this:
1. Hank (protagonist) is setting out to do a scientific experiment. He will dig down through the Earth. If he gets on the other side, then it means the Earth is a 3D object of some sort, so he will prove he is wrong and the Earth is a sphere.
2. If, however, he doesn't get to the other side, then the Earth must be something that isn't 3D.
3. The game is endless, which means he will never be able to demonstrate the earth is spherical this way.
That's how the game is a lie.