Not literal but not a joke, it is a serious phrase of figurative speech. It originates from a french revolutionary "When the people shall have nothing more to eat, they will eat the rich.” which is most likely a reference to the famous quote by the same author, 'At length I remembered the last resort of a great princess who, when told that the peasants had no bread, replied: "Then let them eat brioches.' — Jean-Jacques Rousseau

The rich said let them eat cake, I propose we eat the rich