Most of the North American indigenous languages have less than 500,000 speakers; there are a couple exceptions like Aymara which is on there, and it looks like there’re a few exceptions to the “nothing under 500k” speakers rule, I suppose out of curiosity? But yeah there are millions of languages that didn’t make the cut. American historical linguistics also doesn’t seem to be an area of research that’s had much success as far as cohesive theories or visibility, and as it happens colonial genocide tends to destroy native cultures, so it’s disappointing but not surprising that it’s hard to find evidence.
You’re not wrong, there’s certainly some bias here, but not sure what the intent was, if there was any or if it was just not thinking it through all the way. I know I’ve certainly forgotten components in projects that after review I didn’t know how I’d missed or that in hindsight were crucial.