Irrelevant. The disputed fact was that America was not a country of immigrants. The idiotic assertion raised in the parent post is a refresh of the argument that the Mayflower landed settlers and not immigrants, which is nonsense due to the fact that the bulk of the population of the US are undoubtedly immigrants.
The vast majority of Americans are immigrants.
> Is it a defining characteristic or an incidental one?
Is this even a question? Again, the vast majority of Americans are immigrants.
> I wouldn’t say that’s a defining characteristic that should define our future immigration policies.
You can try to move the goalpost as far as you'd like and blow as many ignorant and ill-informed xenophobic dog whistles as you'd care to blow. You cannot outrun the fact that the US is a nation of immigrants.
If by “nation of immigrants” you mean merely the fact that most Americans have immigrant ancestry, your original post makes no sense. Why would that incidental fact have any bearing on what America’s immigration policy should be going forward? Mere accidents of history don't compel a democratic republic to maintain any particular course.
America is a country that happens to have a lot of people of immigrant ancestry. That’s an incidental feature. Because it’s a physically large country that was for a long time relatively unsettled. But that doesn’t define the country. There isn’t anything that commits the country to any particular policy re: immigration. In fact, the massive German, Irish, and Italian immigration of the 19th century led to a decades-long restriction of immigration staring in the 1920s, where the foreign born population dropped from 15% (similar to now) to under 5% by 1970.