Iacisrmma -I know the basic story, although I don’t remember the details. What I learned was that there was a machlokes regarding where the timeline is halachically. I don’t remember the different opinions, but I thought that one opinion is that the day starts in EY, but not everyone says that, so basically anyplace that is east of EY (such as Japan) is a machlokes (but again, I’m not sure about the details).
In any case, I had heard that when the Jews were in Japan, there were 2 different Gedolim who gave 2 different opinions. I guess one must have been the Chazon Ish (according to both you and Geordie) and one was Rav Tuckazinsky (according to Geordie).
So there were some people who weren’t sure what to do and they fasted on both days to be sure (since you can’t go wrong with that as long as you are healthy enough that it’s not pikuach nefesh, so you are safe according to both opinions). But it would probably be a problem halachically to say Yom Kippur davening both days, so it makes sense as Geordie said that they only davened Y”k davening one day. You can fast as a chumra, but you can’t say the wrong davening or refrain from putting on Tefillin.