According to the vast majority of modern people located on Aldenard, the Lalafell, Roegadyn, Hyur, Elezen and Miqo'te races are considered native to this continent.
They are the five races.
Again, according to modern people, Au Ra, Garleans and any other furture-implemented races are neither of this land, nor hold a sizable population here (no, the millions of player characters don't count) and therefore are not accounted for among its people.
So, when someone refers to the people of present Eorzea, "the five" is a natural synonym.