It didn't matter whether they left the country or not. As long as they resided in the US under the protection of the British government, showing where their loyalty lied, they were still British at birth. Birth on the soil could not be used to gain access to the rights of citizenship and there in lies the crux of the debate. It was formally announced prior to the revolution that parents who were loyal to the revolution became citizens as well as their wives and all their descendants, whether born before or after the revolution. And children born in the US to natives or aliens who made it known that they wished to retain their foreign citizenship, were deemed to be aliens. It all goes to inheritance and children of aliens could not inherit in the US and vice versa. This was the FINAL ruling in Snug Harbor. The child was born prior to the revolution & taken from the country by the father, even though the mother stayed in the US, could not inherit; because prior to Sept 1776 the family moved into NY under the protection of the crown and the mother remained living under the protection of the crown. At no time during the revolution did the father & child come back and move the family back under the protection of the United States. As the court stated, had the parents changed their minds and taken action to throw off their allegiance to the crown prior to the Treaty of 1783, the child would then have become a US citizen. It didn't matter when or where the child was born, on US soil or not, it was the action of the parents becoming citizens that made the child a citizen.
If you cannot read a decision better than that, we have no reason to discuss further. You have TOTALLY misread the decision.