By law, Leo means a law passed by Congress and not a precedent set by the Supreme Court in a ruling that interprets the Constitution.
Congress has the power to naturalize citizens. The SCOTUS does not.
In this regard, SCOTUS can interpret the Constitution, its Amendments, and the constitutionality of the naturalization laws. Interpreting which citizens are affected by a law is not an act of naturalization in any sense of the word. (IMHO)
He also added that "a Constitutional amendment which specifically defines 'natural-born Citizen' more inclusively than Minor did" would also reverse the precedent he says was set by Minor.
If he believes that a Constitutional amendment defining NBC would reverse the definition he gathered from Minor, then I have a hard time believing that his position is, as you said, that a natural-born citizen does not require a law to clarify his status. If he really did believe that, then a Constitutional amendment couldn't reverse the definition of NBC.