Christopher Columbus has nothing to do with Hyrule. Also, what is this about Hyrule being on Earth? There is nothing proving this. Stick to what we know instead of trying to prove a theory with more theories.
365 day years, earth like climates and species, blue skies, a yellow sun, a single moon, and, perhaps most importantly, familiar constellations in tWW make it very likely that Hyrule exists on a variation of Earth.
As for the Flat Earth Theory, it's intriguing, but there's a globe in various areas in tWW, showing that the planet has been proven to be round.