According to the Nenkin Support Center of America (https://us7.campaign-archive.com/?u=ebd ... 7597092e70), it has just received (July 1, 2022) confirmation from the Social Security Administration that Kokumin Nenkin should indeed be excluded from WEP. Apparently, previous applications of WEP to Kokumin Nenkin will be reviewed and refunds issued where warranted.
How exactly this will be handled remains to be worked out. It will probably take some time, but at least it seems that progress is being made and that, in the future, the two parts of a Kōsei Nenkin pension should be receiving different WEP treatment from the SSA.