“Is,” “as,” that’s just parsing of 19th century grammatical expression. The bottom line is “NATURAL BORN CITIZENS.”
What I think we can all agree on is that what it doesn’t say is: “the children of citizens of the United States that may be born beyond Sea, or out of the limits of the United States, shall NOT be considered as natural born Citizens.”
Children born of parents with differing nationalities? Not so much. Such didn't exist in 1787.