Natural born means no law is necessary to determine citizenship. Both parents are citizens and born in country, what else could they be?
If a law is necessary to naturalize, not natural born. Founders didn't want laws being made to manipulate who could be POTUS.
If a candidate would have been a citizen when USA was founded, they are natural born; otherwise some law enacted between now and then naturalized them.
Citizen at birth =/= natural born.