I'd say it's more of a nitpick. You're right, he was more concerned about an English mythology rather than a European one. But I'm not sure where you're going by bringing up North America...
I thought OP was saying that Middle-Earth was Europe. Apparently, that wasn't his argument. But I wasn't going further than what I said: view Tolkien's map of Middle-Earth in a mirror and you will recognize the coastline and mountain ranges of North America. It is not exact, but it is too similar to be a coincidence. Tolkien did not randomly draw Middle-Earth in that shape, he used a mirror image of North America as a starting point to create the map of Middle-Earth.
I'd say it's more of a nitpick. You're right, he was more concerned about an English mythology rather than a European one. But I'm not sure where you're going by bringing up North America...