His parents were legal residents of the United States, had already begun the (time-consuming*) process of getting citizenship, and were subject to the jurisdiction of the United States.**
"Anchor baby" refers to a child born in the US to a foreigner without resident status, who then uses that birth to attempt to gain legal residency in the US.
Rubio is not an "anchor baby" (though he is the chief beneficiary of this DumbTrumpDump). Try again.
* "time consuming" because you have to file all your papers, state intent, and then wait a minimum of 5 years. His parents were naturalized in 1975, Rubio was born in 1971, obviously they had already sworn as to their intent before he was born.
** "subject to the jurisdiction of the United States" means they had no diplomatic immunity and could be tried in American courts just like you or me."
They were not yet citizens.