It does not mean that her lineage was unchanged from the ancestral founding population while other populations evolved.

It does not mean that she is the ancestor of all other Native Americans or that her lineage is in any way superior, inferior, or more authentic than other Native American lineages.

It means something much more specific and much more modest: that when the founding population of the Americas began to differentiate into distinct subpopulations, the lineage leading to USR1 was one of the first to diverge, and the populations sampled for comparison had diversified further after that initial divergence.

To make this concrete, imagine the founding population of the Americas as a single ancestral group.