If I think back to it, Trump has a historical hatred for Japan but a more neutral perspective on South Korea. Likewise South Korea has higher value to the US now by virtue of Samsung, SK Hynix, direct US control of their forces, and the naval shipbuilding arrangement. In contrast, Japan is all strung out and has nothing to offer but basing, prostitutes, and tourism. It would make sense to me that if the US was going to offer a sacrificial lamb as part of the rare earths surrender, it would be Japan.