400-year-old traces would be hard to detect owing to admixture, but if they could find identical-by-descent segments that would be very compelling, as the research into Native American traces found in Polynesian populations shows:
https://gizmodo.com/native-americans-voyaged-to-polynesia-lo...
If Croatoan ceremonies didn't involve cremation it could be quite a bit easier.. I don't really see the article's evidence as very compelling. Many things may have been collected from the site and ultimately discarded in the trash heaps without the proposed integration.