OK, I give up. Why isn’t Rubio a NBC?
His parents were still Cuban nationals at the time he was born.
Children of foreign nationals inherit the nationality of their foreign national parent(s). Obama was born a British subject because of his Kenyan father , he is not a natural born citizen.
One is only a natural born citizen when one cannot be anything else.
In the unlikely event Rubio’s parents had returned to Cuba, he would have been a Cuban citizen.
He is not a natural born citizen.