I should mention that it’s not like they utterly deny they ever did anything wrong in WWII/Second Sino-Japanese War. It’s one of those “we did bad things and we’re collectively very sorry and regretful about it” without ever explicitly acknowledging what any of those things actually were. And since some of the specifics are so bad that they kind of fly straight past “we did some bad stuff” they can’t believe that such a thing could have really happened and that the country wouldn’t be sorrier about it – and thus you’re making it up to make Japan look bad. Because if Japan really did something that bad, they’d know about it and try to make amends.