The two countries have been at odds in recent years over Japan's invasions and occupation of China in the 1930s and '40s. China has accused Japan of not fully atoning for its aggression, while some Japanese feel accounts of their wrongdoings have been exaggerated.Look, some Germans still don't believe the holocaust happened. There's no reason to give them the benefit of the doubt in an international wire piece.
The larger problem is not that "some Japanese" don't believe the accounts of WWII atrocities, but that those same "some Japanese" frequently have prominent roles in Japanese government -- such as Prime Minister. When the Chinese say that Japan hasn't fully "atoned" for it's aggression, they're on far more solid footing than the Japanese who say nothing happened.