Actually, the law at the time suggests otherwise.
His mother was 18 at the time, and there’s a time in the US issue. He may not, technically, be legally US by birth, even if his mother was a US Cit. . .
That’s a stretch. I don’t think that law applied to someone who was born, raised in, and never resided permanently outside the United States, even if she were only 18. Heck if that’s the case then millions of Americans who are 4th or 5th generation, born to mothers under 21, aren’t “natural born” citizens.