Occam's Razor. It was most likely a lost human from a distant land, speaking another language, traveling in an exotic looking vessel. This is a simpler explanation than invoking extraterrestrial life.
Very much agree that the UFO thing is a red herring. But perhaps Occam's Razor actually points to an even simpler explanation, that of it being a reoccurring mythological trope that was repackaged as an historical event.
That said, I would love if it actually were the case that a mysterious woman from, say, the Dutch East Indies — to make up a random, semi-plausible example — really did wash ashore in a Japanese fishing village. Sounds like a sequel to "The Thousand Autumns of Jacob de Zoet" (awesome book by the way).
