Those people weren't "citizens from birth" of the nation they lived in. No one is arguing that people who weren't born citizens are eligible for the presidency.
The Founders were definitely aware of a "Manchurian candidate" issue with the Presidency.
The Manchurian Candidate was a natural born citizen by even the strictest definition.
No, you are arguing the converse; That someone born in a nation, yet raised in another nation will still have loyalty to the nation he was born in rather than the nation he was raised in. In either case, the loyalty is suspect. Not so for people born and raised by loyal parents in their own country. Can you not see why you wouldn't want such a situation? So did the founders.
The Manchurian Candidate was a natural born citizen by even the strictest definition.
Technically perhaps, but not in spirit. The notion of a disloyal fraud is still apt.