Yes I agree completely in one aspect that Japan as a whole is not terribly diverse, but places like Tokyo are more diverse than the grand majority of the United States by magnitudes, possibly with the exception of a few major cities in the US itself.
So what I'm saying is that his pre-concieved notion of what Japan has to offer is offensively false because you can actually get more genuine international interactions over there in the following fashion. Let me just share some examples of this.
1) When I moved to the US, I was shocked that I can't watch non-American movies at movie theaters on opening night, because that concept doesn't even exist. America has some dictatorship (not literally, its tongue and cheek) of sorts with film distribution. That's not diverse, or free. People in the US are deeply opposed to subtitled movies with only a few exceptions. That's not diverse, that's..... I dont even know what to call it.
2) Cuisine. A lot of US food masquerades as international cuisine, yet isn't at all. Like American Chinese food. None of that is practically authentic, let alone Chinese. The Italian food in the US is all watered down versions of Italian cuisine. In Japan Tokyo, you can literally walk into a run down tiny Indian/Russian/Actual Italian eatery and eat real food, often with the cook owning and cooking the food right in front of you, and even strike up a conversation. I've never seen this sort of heart to heart establishment in the US before. I can go to hundreds, if not thousands of bakeries in Japan and have fresh bread of different kinds. In the US, my best shot is Panera bread unless theres a local bakery, which are increasingly harder to find.
Heck, I had a German-American look at me all crazy when he didn't know what a baumkuchen was and it took him a hell of a time to try it because it had "asian writing on it". Meanwhile my German friends in Japan would love that theres plenty of baumkuchen to go around, so theres definitely this disconnect between what people "think" is authentic or cultural and what actually is.
3) Events. Japanese people, for lack of diversity, have a lot of international populations. 1 out of 11 childbirths are biracial births in Japan now. That's almost 10% of the population in the near future, coming out of a population thats supposedly homogenous.
I'm not saying Japan is a perfect place, I'm just saying that its different. Honestly, if you want good Parisian bread, I have a better chance at getting it in Japan (not just Tokyo, but Japan) than I do in the US conveniently.
On the other hand, you could kill someone in Japan and not find a good Taco joint. But man, you want a good Paella (like, Spain Spanish food?) You will find it with ease?
Want a good Newyorker style pizza? Sorry, you're screwed. Want a pizza straight out of Naples, Italy? Japan's got you.
My point is that Japan is different.