I don't think that follows at all. The fact that an International Statute of Secrecy was not yet in place implies that international cooperation among wizards was probably still nascent. Looking at early American history and who colonized the New World, I think it's
When Jamestown was founded, the Statute of Secrecy wasn't even in place yet, so wizards would have come along with Muggles openly and looked for a new life in America.
Actually, I think she almost certainly didn't think it through. A list of Quidditch teams is hardly canon about wizarding governments. Why are states on the same level as countries? Is the complicated arrangement of Scotland, Wales, Ireland, and England under the United Kingdom reflected exactly in the wizarding world? Would purebloods, Death Eaters, even, care about Muggle-established boundaries? Rowling never even mentions the United States by name in the books -- "American" witches, yes, but there's no evidence that American witches and wizards consider themselves part of the U.S. Did Hawaiian wizards suddenly consider themselves Americans when Hawaii became a U.S. territory? A state? When China became communist, did all the Chinese wizards, too?
Considering the number of countries that JKR names directly, I think we have to assume that she imagined wizards to live within Muggle borders. And as I already said, just saying 'she didn't think that through' or something like that won't do. She did list countries, and rather a lot of them. They are the same as in the Muggle world. If we are fanfiction writers, we have to accept that. If you're going to write an AU story, it doesn't matter at all, of course.