A bit worse than wrong because your postulating 'data' that fairly clearly indicates the opposite of what you are saying. Also there are some serious misunderstandings here.

China is 'a free society' and 'internationalist'?

(Han) China has always been the opposite of 'free', which of course is a modern concept, that's not a judgement, just what it is. And they have not really ever been 'internationalist' only just recently joining the internationalist order, and vaguely so.

But that aside:

China, Japan and Korea, if we were to apply our reasoning objectively to them as we do ourselves, are objectively 'far right ethno-nationalist' states.

They are technically, quite fascist. I wouldn't want to use that label in normal discussion as I don't want to trigger or flame war, but in all reality, this is the fact.

All three have very, very strong national narratives about who they are, their origin, their superiority (to greater or lesser extent). They are ethnocentric culturally (i.e. a Black person in Korea might be very respected, but nevertheless considered 'not Korean').

Their religion, elite, and culture tie together, literally in Japan with the God Head Emperor, and in all three places elite families ruling by long held extra-governmental power networks.

'Samsung' is not a corporate formation that we would understand in North America, though Europeans might understand it a bit.

Japan and especially China historically have conquered and assimilated vast territories in the vein of de-facto superiority, some of this goes on today in E. China and Tibet.

This is not a condemnation or anything of the sort, it's obviously complicated, it's just what it is.

But those nations are the furthest thing from 'internationalist' in anything but the modern, economic sense with respect to trade and some institutions.

Europe, historically, has had fuzzy political lines, but they bounce around and generally land along ethnic borders.

Yugoslavia did not break up into random political territories, rather, ethnic one's. Same with Hapsburg Empire. Same with Soviets. Etc.

Scandinavia is not a hodge-podge: most of the ethnic Swedes live under the same flag. Not all, but most.

Obviously, especially on the continent, it's not so clear - yes - German speakers are scattered a bit in Poland, France and Czech - but they are overwhelmingly in Germany/Austria. That there are lot of 'fuzzy lines' doesn't mean lines do not exist.

"The internationalist world has produced freedom, security, and prosperity at orders of magnitude beyond what humanity ever experienced before. It seems bizarre to me to trade that in for the old nationalist ideologies."

This statement implies a misunderstanding of what 'Internationalist Order' and what it means.

The 'Internationalist Order' is literally about 'Nation States' cooperating in some reasonable, common ground for trade and security. If anything, that 'order' is predicate upon the very notion of Nation States, it's literally part of the definition.

'The International Order' reinforces the notion of Nation State, and indirectly, nationalism.

It's literally in the word: 'Inter'+'Nationalist'.

Canada, US and Brazil are probably the leading examples of what 'pure' nation states are, i.e. not fully ethnocentric, and culturally more plural. I do believe there are material advantages here, but 'net net' I'm not so sure they are in the long-long term, I think it's more of a wash.

But even with that plurality 'Nationalism' is still quite strong: paradoxically (or maybe not) 'Overt Nationalism' is stronger in Canada and the US than it is in Europe, because of weak local social ties, it is actively encouraged in North America. Germans (and Han Chinese, Japanese, Koean) don't need a flag to know who they are, flag waving for them is something completely different as we learned in the last decade or so where they are finally allowing the 'don't wave flags because we don't want to seem like Nazis' taboo fade a bit.

I think the OP you are responding to is essentially correct, but the future is so much harder to predict.

Even as we travel more and are exposed to more, we have more well-defined and managed borders than ever before, and there are few places to just 'go to', so maybe there will be less diversity in change than there was before? It's so hard to say.