This is a bit of a turning point, I think, In our understanding of Yonaguni. It's not man made at all... Yonaguni is volcanic rock mixed with heavy liquid sediment of 'Alkaline Earth Crystal'. This is apparent by a simply comparison. I have not yet had a 'geological' opinion regarding this, yet it would seem that geologists are stuck in the paradigm that 'rocks are not crystals', which is incorrect. In some ways all rock is a conflomerate and I suppose this is what they mean, whereas they hold a crystal to be 'pure'. Actually no such thing exists in the chemical world as a pure crystal, and I have a chemistry background.
Yonaguni appears to be a rock, perfectly natural. But that exactly does that mean? In particular I've suggested it looks most like an alkeline Earth Crystal. I'm suggesting it's 'crystaline' which means it's a relatively pure form of rock. Other rocks on earth contain bits and pieces of everything, different crystals and hence have a more 'rocky' and less 'crystal' finish. This is different. It's one pure formula repeating over and over. It appears to have simply recently erupted out of the mantle (Pacific ring of fire) and then spontaneously formed as a result. This sort of artificiality can simply form spontaneously by nature.