So then if citizen Cruz isn't natural-born, then when was he naturalized?
Cruz was made a citizen by statute.
As far as I know, he wasn't. If I recall, his father wasn't a citizen until 2005, well beyond the point that derivation would have been able to play a part in his son's citizenship.
There is nothing in Cruz's citizenship to keep him from holding any public office except the President or VP. Having admired his knowledge of the Founder's intent and having voted for him twice, I must say I'm disappointed he has made such a statement.