There is always a hierarchy. Otherwise, there wouldn't be juniors, seniors, managers and C-suite. Each of the higher roles come with different responsibilities and also things they can do. Can you really do hiring decisions or build the next strategic plan for the company at junior level? No. And there are many more things like that.
Open door policy is more about being able to put across your points to anyone directly, say to the CEO, without going in steps. That hierarchical structure is still there but people just don't need to wait for their manager to tell their manager and then to the CEO and so on. Still, in larger companies you may not be able to directly get in touch with the executives which wouldn't be open door policy.
An example much closer to "no hierarchy" company would be Co-Operatives where every person is an equal stakeholder in the company.