Why is macroeconomics about general equilibrium? In a fairly popular paper published a couple of years ago ("On the future of macroeconomic models"), Blanchard (2018) makes the following statement, among other "widely believed propositions":


  (i) Macroeconomics is about general equilibrium.


Unfortunately, no note or reference is given for this statement.

Is this statement true? How can it be formally proven? What is the intuition behind it?
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.