Earlier quoted context omitted.
What do you mean? Native American Indians are exactly ethnic Americans. The rest are from other ethnicities. Just as there were (are) ethnic Romans.
Ok, but the vast multitude of Native American tribes absolutely identified as distinct ethnicities from one another. If you were to apply this to the Romans, then "ethnically Roman" would include Greeks, Celts, Berbers, and dozens if not hundreds of other ethnic groups, each of which was represented in all the social strata of the Roman state at various times.
The native Americans would absolutely consider themselves as more similar to each other than to the European and African arrivals. Just by matter of undeniable physical features.
As for the Romans, it is within the mists of history and mythology (Rape of the Sabine Women), but nobody can deny that there was an original people there in Italy, and other peoples who became citizens and/or subjects of Rome weren't ethnically Roman.