I fixed that comment for you. With the Sioux we would have an appallingly representation of North America, The United States and peoples in bumped into. The Iroquois were a power in their own right but there were a great many others before the US was even a twinkle in a colonialists eyes.
I have to say this strikes me as a pretty daft argument. At the moment, the North American continent has representation by four civs. Half of those (Aztecs, Maya) have nothing to do with the United States, temporally or geographically. Meanwhile, there are fifteen civs from Europe known so far, counting Poland and the return of Portugal. There are three (Huns, Carthage, and, as implemented, the Celts) who are only in the game because they had a connection to the Romans. There are two from the island of Great Britain (England and, again as implemented, the Celts), which makes that island considerably more over-represented than North America when the relative sizes of the regions is taken into account.
Historically, they've been very bad about putting in Native groups from what is now the United States. The Aztecs, Mayans, and Inca have appeared pretty consistently, but no Civ game to date has included more than one Native group from north of Mesoamerica. Civ II had the Sioux, Civ III had the Iroquois, Civ IV had the lazy and token "Native Americans", and Civ V has the Iroquois again. Meanwhile Europe gets more and more crowded every game.
Even if there are three Native American groups added for BNW, that would bring the total for North America up to seven, which would make it tied with the Middle East/North Africa region. (Actually, if it's true that Morocco is in, that would still put the Middle East/North Africa at one civ more than the entirety of North America.)
North America is not over-represented. The United States is not over-represented. If any region is over-represented in Civ, whether Civ V or any other version, it is and has always been Europe. North America is empty by comparison.