As I posted the first time this same story was posted here (earlier today), there was no “America” yet and even if there were the Indians had slaves before any white Europeans came to this continent. So the headline is wrong twice over.
Arguably, the headline contains the qualifier “legal” - don’t think there was any (codified) legal system on the American continent before the English et al. settlers brought that with them. Tribal customs probably don’t meet the standard of a “legal system” as we would understand it.