Two reasons. 1) as his parents were legal and not on temporary visas, he could be birthright. Doing away with anchor babies and illegal alien births as qualified for citizenship would not end birth citizenship for legal non-citizen residents.
2). If the above were not to apply, he could become a naturalized citizen.
As I understand it, without birthright citizenship only children of citizens are automatically citizens. The 14th amendment didn’t make the distinction of legal or illegal regarding aliens. The author of the amendment specifically says that.
For example, a Japanese tourist here legally and giving birth to a child would not be a citizen.
So if Rubio’s parents were not citizens when he was born, which I thought was the case, he would not automatically be a citizen without birthright citizenship through other means.