It's along the same lines as:
[Hears X is from country Y] "Oh do you know bob?"
whether he is indian or he only knows about Indian culture, it is a stretch to say "so you must know about Japan!"
Which is an entirely reasonable question, because if X from country Y is at the same party as you, then there's a good chance he might know your friend Bob. It's a small world, they say.
In 1982, however, immigrant Indian and Pakistani businessmen, looking for low interest SBA loans and affirmative action in government contracting, talked the Reagan Administration into reclassifying them from white/Caucasian to Asian/Oriental, even though grouping Indians with Chinese rather than with Afghans makes little sense from the standpoint of physical, genetic, linguistic, or cultural anthropology.
What was it about India that made you think it made sense to ask about China and Japan?