It means that the area is as open as the land. If you think about it, land is pretty open. It basically means it's as big as the whole entire amount of land on Earth.
When we say that something is as open as the land, it usually means that out of the possible outcomes, it could go either way. For example, if we say that the future is as open as the land, what we mean is that we do not know what the future holds, in the same way that we do not know what the land contains. "Open land" refers to vast amounts of land that has not been explored or settled.
