Why is macroeconomics about general equilibrium?
Not sure what you mean by formally proven in this case, but it’s just true by definition.

Macroeconomics is by definition branch of economics that studies the behavior of economy as a whole as opposed to individual markets or actors separately which is studied by Microeconomics.

General equilibrium is by definition an equilibrium of whole economy or several markets.

Hence it just follows from definition of both general equilibrium and macroeconomics. Most of the economics is devoted to study of equilibria for better or worse so if you study macroeconomics you almost always study general equilibrium in one way or another. It’s equivalent to say that microbiology is about small organisms. It’s true just by definition.