Claiming that this affects "10% of all Yahoo accounts" or that the breach in June 2012 is related is disingenuous---Yahoo and Yahoo Japan connected in brandname only ("Yahoo" accounts are not valid "Yahoo Japan" accounts, and vice versa).
Another article clarifies that the total number of Yahoo Japan accounts as 200 million, so it was actually 10% of all Yahoo Japan accounts. My apologies for contributing to that impression.