I did not see how Donofrio was writing here to define what ‘natural born citizen’ is, but rather that is something more than being born here or as you refine as something more than simple citizenship. Rather at the very end of his piece he refers to Minor v. Happersett where it is unequivocally determined that ‘natural born citizen’ means a child born in jurisdictions of the United States of two citizen parents.
I also failed to see any effect of your point of ‘simple birth’ contrasted to ‘simple citizenship’ other than a splitting of hairs or perhaps at most an addition of precision where none was really needed since Donofrio is not writing statute but merely drawing attention of the rule of ‘statutory construction’ to lay readers as a rule that every lawyer knows well as a basic of law.
Natural born requires something more than being a citizen. Being a citizen who was born in this country may be that "something more". If he wants to argue that "something more" has to do with the parents' citizenship, that is fine. But the plain language of Article 2 and the 14th Amendment do not LOGICALLY prove the statement Donofrio is making. Neither do they disprove it.