It is a theorem about NBG because it is proved in the metatheory by induction on NBG formulas, and its proof describes how to use NBG axioms to construct a class satisfying a given formula.
It is a theorem about NBG because it is proved in the metatheory by induction on NBG formulas. Also, its proof--instead of consisting of NBG axioms--describes how to use these axioms to construct a class satisfying a given formula.