Yes, you could have a corporation that does not have an internal hierarchy. But (in a left anarchist view) you cant have a corporation without private property and corporate limitation of liability. Both of those are (in their view) hierarchical.

The problem from a left anarchist standpoint isn't (just) "CEO is boss, having boss bad" its that there's a group of people with special treatment everyone else is not subject to.

You might disagree with that view, and probably do. I am just familiar enough with their ideas to relay and explain them.