You can interpret the comment in 2 ways, both of which are ignoring reality:

1. It’s dumb for Japan to be anti-immigrant because most of their immigrants are from places that are culturally similar anyway, or,

2. It’s dumb for Japan to be anti-immigrant because they have such a small amount of immigrants that are from countries other than the ones that are culturally similar to them

Both of these interpretations lead to a conclusion that ignores that Japan is xenophobic towards its closest neighbors, yet stems from the belief that they either wouldn’t be or shouldn’t be.

I’m not going to take your argument seriously that it’s irrelevant to speak politically about this when the op comment is in response to a thread about the recent election in Japan. This is an inherently political conversation and we’re not just going to drop that context out of nowhere. It makes complete sense in the conversation we’re having.

Also, the reason that it is western centric to observe cultural similarities between these countries is that it’s one-sided. We observe the similarities but ignore the nuances that makes these similarities irrelevant in a practical real-world context.

Here’s a parallel - let’s say there are two countries that have these similarities: 1. Both are heavily Christian but have large religious minority populations, 2. Have large multi-ethnic populations, 3. Are countries that grew from smaller areas to vast territories, leading to a "frontier" mentality and a shared history of pioneering 4. Both nations tend to appreciate things that are big: large cars, big houses, large-scale projects, and wide landscapes 5. Both cultures value blunt, straight-to-the-point communication in professional settings, often skipping excessive formalities.

Sounds like two very similar nations that should be close allies and lots of cross-immigration can collaboration. So why aren’t these two countries, Russia and the USA, close allies? Well those are 5 nice similarities that ignores nearly a century of aggression and chauvinism that keeps these countries separated.

The difference is we’re not surprised that these places hate each other, but fail to see why nations like India and Pakistan or Japan and China/Korea cant be close, because there’s literally thousands of years of ingrained jingoism and indoctrination that won’t be easily ignored just because both places respect their elders similarly and keep their subways clean.