The depiction of the natives has been something I've been curious about since I got into the series. Now that I've seen it, I'm pretty impressed with the amount of thought that went into them.
Based on a quick search, I think they are based on the Miꞌkmaq, a first nation people who currently live in the area where Vinland is supposedly located. There isn't much evidence that they ever made contact with the Norse (since the "Skrælings" in the Sagas are speculated to have been the proto-Inuit Thule people), but the geography seems to fit and there isn't any hard evidence of who the indigenous people of Vinland truly were.
Nevertheless, he clearly did his research when going into this topic. Accuracy can be debated, but his attempt was definitely not half-arsed. Well done, Yukimura-sensei.