It has been for the whole time since entry policy shifted from default-allow to default-deny (a change itself largely based in racism, notwithstanding that there are good reasons to want to avoid a large, permanent, non-citizen population); when the US had open immigration and only restrictive rules for naturalization, yes, mass illegal immigration wasn't an issue. [0]

> It is not a racist thing, it is a political thing.

That's like the “it was not about slavery, just states rights” line about the civil war; and just like that it sets up a false dichotomy to pretend that the (true) more general description falsifies the (also true) more specific description.

[0] I want to emphasize that this is not advocacy for a return to such a policy, only noting that the US has never, for any significant time, had a working restrictive immigration policy without significant illegal immigration.

Not really. Colonization is a very different phenomenon. So is conquest.

The indigenous Americans didn't organize their societies as nation states.

So, I doubt that they had "borders" in the sense used here. Supporting this is the fact that their cartography wasn't very advanced. Here's how their maps looked like: https://www.archaeology.org/issues/338-features/maps/7549-ma...