If Mt Fuji is a word, then I demand to know its antonym.

This might sound like a blinding thrust into the obvious, but a word is anything which is intelligible to the reader or the hearer as a communication with a meaning which is understood, or which is understandable by studying the language or culture of the communicator.

Therefore, "Mt Fuji" is a word, or perhaps two words, and it matters not if it has an antonym or an unclnym. It is a word in any event, or, more precisely, it is two words together producing a name, and that name is a word.