>This isn't true and it ignores the cultural differences amongst all of the original colonists (religious, language, political, and country of origin).

You got it backwards. This is BASED on the "cultural differences amongst all of the original colonists (religious, language, political, and country of origin)". That was what made the place have no "prior historically established dominant ethnicity or culture the kind other nations have had going for millenia".

>That's before you even consider the stark differences in culture between the Native Americans and the European colonist.

Those would be relevant for explaining the "American melting pot", if the latter wasn't established after erasing both their culture and, to a large degree, them.