Here’s a simple chart of Japan’s treasury holdings(Source: US Treasury); they just keep adding to their pile. They are at $804 billion now. If the BoJ didn’t absorb the proceeds from the Japanese trade surplus with the US that would create a consistent upward pressure on the yen vs the dollar.
Now that the PRC is using some of the proceeds from its trade surplus to purchase Japanese government bonds (Reuters), the Bank of Japan has to find a home for the proceeds of the PRC purchases. Perhaps those are invested in Treasuries as well. The PRC’s recent turn to Japanese debt may become a diplomatic issue as well as an economic one.