It couldn’t show even that.
There may be other explanations, but I can’t think of many at the moment (other than war/disease wiping out most of the North American population and leaving only one group standing). Mitrochondrial DNA acquires harmless mutations at a more or less constant rate. If the appropriate number of harmless mutations are found in all native Americans and not in other populations, that is significant evidence of genetic isolation in a small, well-mixed population. What is your alternative?