Presumably they took Miramanee's ancestors because they saw that the Native Americans were endangered by European diseases and colonization.
If that was their intent. But if the preserver's desire was to establish an intelligent species upon a world that had none, and they were somewhat limited in the numbers people (and goods) they could move, then relocating a primitive hunter/gather people would make more sense than relocating a somewhat more advance people, like those who existed elsewhere on Earth at the same time period.
Miramanee's ancestors could have been basically "dumped" in the middle of a lush forest with little more than the clothes on their backs, and have been reasonable expected to survive and prosper. Over the course of tens of thousands of years, Miramanee's people would populate the world and eventual perhaps become a technological people.
As pointed out, if helping Miramanee's ancestors was the primary aim, then that particular world (one needing a deflector) was a poor choice. If helping Miramanee's ancestors wasn't
the goal, then filling a niche may have been.