I am aware of the Ainu, though I'm certainly not an expert on the topic. Wikipedia suggests that there may be 25k to 200k Ainu people in Japan, which would be 0.02% to 0.16% of the population; and supposedly the language is nearly extinct. So my point about Japan being monoethnic "for all practical purposes" still stands, as Ainu culture is not widespread enough to make a difference.

I suspect the broader point you're trying to make is that the concept of Japan (or any other country) as a monoethnic country is in many ways manufactured by politicians and other powerful interests over time; and in many cases literally manufactured by genocide. And I grant you that's entirely true, potentially in every single "monoethnic" country. But, manufactured or not, there is a tangible difference.

I myself grew up in a monoethnic country, with the presence of a unifying cultural common ground; for example, I could rely upon the fact that a popular television show was being watched by nearly everyone in the country, even in the remote corners. And that's just one minor example of what adds up to the sense that yes, this stranger next to me has had a similar life to me, at least in what we were taught in school, the media we consumed, the food we grew up eating, the behavioral customs we expect of each other, and so on.

The absence of this unifying cultural common ground is the characteristic of a polyethnic country.