If that is so absolutely unerringly obviously true, then why did it take until 1924 for an act of Congress to make Native Americans citizens of the United States? As you state it, Native Americans would have been considered citizens by virtue of the fact that they were born "within the borders of the US, including any territory or possession" but they were not. If Congress can declare that Native Americans are or are not citizens, then Congress can declare illegal aliens to be citizens or not by dint of legislative fiat, just as they did for Native Americans can they not? If they cannot, what's the Constitutional difference under the 14th Amendment between a Native American non-citizen in 1918 born in Oklahoma prior to the enactment of the Indian Citizenship Act of 1924 (who would not have been considered a citizen) and an illegal Mexican alien born in also born in Oklahoma in the same time period?
Congress merely ratified what had always been the law. Congress cannot create constitutional law. I presume in 1924 there was a question of sovereign indian nations claiming their members were not being accorded equal protection of the laws. In response Congress removed the issue from the prospect of the judiciary. Your example being used to prove a negative would not be persuasive to any legal authority.