The Aztec Empire was founded in 1428 by people who migrated from the north to the Valley of Mexico.
By your reasoning, the Aztecs should not be counted as the indigenous people of the Valley of Mexico. They certainly are considered as such.
There are two ways of looking at your argument:
Consider the Aztecs narrowly as a fully separate and distinct people. In that case, no, they don’t count as “indigenous” because there were other peoples (e.g. Teotihuacan people and Toltecs) there before them.
Consider the Aztecs broadly, meaning you’re really talking about the Nahua people as a whole. Then yes, they do count as “indigenous,” but were also there way before 1428.
You don’t get to have it both ways, with Schrödinger’s “indigenous” being simultaneously the first and not arriving until 1428.
Your argument is like claiming that the Romans were the “indigenous” people of central Italy and have been there since 753 BCE and not a minute before, because (for some reason) the Latins and Sabines (and the Italic tribes they descended from) don’t count.
Here’s a question for you: who are the “indigenous” people of the Falkland Islands? Is it Europeans, or nobody?