Here's another way to look at it: the economy is the process of producing/acquiring goods/services; the political process is the process of effecting choices for the community as a whole (any community, from a book club to a nation-state). The structure and sanctioned functioning of the economic system (the set of institutions through which the economy as a process proceeds) is a choice effected for the community as a whole.

To be sure, throughout the history of civilization economic power has translated into political power, bending that choice to the interests of the economically powerful. Even so, that is much different from making economic relations the foundation of all of human culture. Ideas do make a difference, to include ideas that lead to a different economic system. The socialist revolution in Russia was, after all, a political act.