Mussolini's use of "corporate power" and an American's understanding of "corporate power" are not the same thing, people quoting Mussolini that "fascism is marriage of corporate power and the state" don't really get what he's saying.

Under fascism, a corporation is kind of equivalent to things like worker's soviets in the USSR.

You have a corporation of workers, a corporation of peasants, a corporation of industrialists, and even, believe it or not, a women's corporation.

The fascist "corporation" is a way of formalizing various interest groups within the infrastructure of the state by giving those interest groups direct representation in a sort of "parliament of corporations."

Hence, "everything within the state, nothing outside the state."

The American understanding of corporation is simply "a collective entity formed for the purpose of capital accumulation and stakeholder profit," whereas in the fascist understanding a corporation is a collective entity which exists to advance the interests of a particular class.

It's a subtle, but important, distinction.