I read it as criticising reductionist views of the many diverse nations that existed in North America before Europeans showed up and decided that the whole continent was Terra Nullius.

To this day a significant number of US high school American History textbooks only discuss the tribes in terms of their interactions with European invaders, and shy away from anything that might make them look like they were ever legitimate nations. Referring to them as 'Native Americans' instead of by name also has this effect.

I believe it's the verb tenses. Instead of it being a historical fact, it's an ongoing practice of an ongoing group of people

I think that's why there's a smile in the last square.

Well, “Native Americans” means everything from whoever lived on the tip of today’s Argentina all the way to the Inuit. So saying “native Americans” when it’s actually just two tribes is wrong.

Edit: Wikipedia says the technique was used by ‘various’ people.

There is a tense change. The second person is saying the technique is still used by those tribes today.