I've always held that the extremes of corporate power look a lot more like feudalism than anything else.

The main difference being that corporations divide up the different experiences of life, where fiefdoms divided the land. In the end we have absolute rulers in their own private domains who only have to pay a small tribute and lip service to the nation they are nominally aligned with.

In theory Communism is rule by the proletariat, monopolistic Capitalism is rule by a very few super-rich shareholders/owners. They seem like markedly different situations to me.

Can you explain further?

Most publicly avowed communist countries actually adopted State Socialism. And in practice, these systems of state-owned industry ended up under consolidated control of oligarchic regimes. Some communistic critics of these Marxist–Leninists regimes even called them State Capitalist. It is not uncommon to refer to "Communism" and mean some form of despotic, top-down state control of industry.

Given that revised definition of "Communism," a greatly consolidated, monopolistic Capitalism, with increased favoritism by the state for leading firms, does start to seem similar in some interesting ways.