I agree that the assumption makes no sense, but the calculation offering an 80mi diameter elsewhere in the thread is, in fact, based on distributing the Earth's

I agree that the assumption makes no sense, but the calculation offering an 80mi diameter elsewhere in the thread is, in fact, based on distributing the Earth's land area across the number of tiles in a (Civ 6 huge, IIRC) map, calculating the area of each tile based on that, and then calculating the distance across the tile based on the area.