He was born in the US to persons in the country legally, so he was born a citizen, but not a natural born citizen becasue his parents were not yet citizens themselves.
His bio from Wikipedia:
Rubio was born in Miami, Florida, the second son and third child of Mario Rubio (1927–2010) and Oria Garcia (born 1931). His parents were Cubans who had emigrated to the United States in 1956 and were later naturalized as U.S. citizens in 1975.
"Citizen" and "natural born citizen" are NOT the same thing.
Sigh! I hope Rush soon weighs in on this INSANITY but until then there are only TWO kinds of citizens: Natural Born and Naturalized. One or the OTHER. There is no in-between citizenship purgatory.