The Act itself wasn't unconstitutional. The US Govt, in that Act proclaimed to the world and all US States that “jus sanguinis” encompassed with exclusive allegiance either at birth or naturalization was the guiding force behind the definition of US citizenship. As I said, what they didn't contemplate was foreign nations under feudal law ignoring the right of expatriation and therefore claiming children of foreigners to be theirs as if those feudal governments were the parents. This is where the treaties come in. “Natural born” was removed in 1790 to avoid further embarrassment due to the existence of feudal law in other countries. Natural born is “jus sanguinis”, however “jus soli” may come into play depending on the birth location of the child. If the child was born in a foreign nation that did not rule under the feudal law of “jus soli”, then there was no question.
The question only came into play for those born in countries under feudal law and that is where statute law applies. They were/are children by statute because at the moment of their birth, they were considered members of a foreign nation. The are children who could never be left stateless if the US denied them citizenship because they already had an existing foreign citizenship.