I believe the sentence is supposed to be read this way:
The founders intended that a only natural born citizen be president. However, "at the time of the Adoption of this Constitution" (1787), the founders themselves had all been born British subjects, even though they may have been born on this continent. They did not consider themselves natural born citizens, so needed to exclude themselves from this rule in order to have men eligible for the presidency in the early years.
Rubio was not born in the late 1700's, so he needs to be more than "a Citizen of the United States" to qualify - he needs to be a "natural born" Citizen of the United States.
The fact that his parents were not American citizens when he was born here, calls into question whether or not he can be considered a "natural born" citizen. This is about dual loyalties. Traditionally, citizenship is conferred on a child through the parents, or just the father. This point can be argued, but it is wrong to simply ignore the Constitution's intent or to ridicule those who point out the fact.
That said, the establishment prefers to distract from the citizenship issue with misinformation and ridicule. I have little doubt they'd be happy to simply legalize swapping our own corrupt leaders with those of other countries, making it easier to maintain a single, ongoing power structure worldwide. (Kind of like it seems now.)
This is certainly not the best issue to lead an attack with - and there is so much other evidence of this man's incompetence to focus on, I say drop it... at least for now.