At the start. Didn't the U.S, then not yet united, have 100% foreign born residents?

That is a myth. Americans were already locally born for couple of centuries before they declared their own independence.

I referred to the colonists. It cannot be a myth that they were by definition all foreign born.

We don't have a clear ratio of how many were born in the Americas, when Independence was proclaimed. I concede the majority was likely born on that soil. I find that irrelevant to the parent comment stating immigration was controlled in the early 1900s.