Nested Circles is a hierarchy and Holacracy is never about flat structures. This is a misconception that the Holacracy folks have been trying to clarify since the very beginning.
Now, if you can separate for a moment, the people from the roles they hold, then the people themselves are flat, as there is no one person above or below another. They simply fill roles in different circles, some with smaller scope, some with bigger scope, sometime deciding direction, sometime following, but never above or below. People are never owned by a circle. They do not need to get permissions to participate in another circle or resign from one, although they may have to follow an agreed upon procedure for doing so.
Beyond the structure, the fact that any role (and thus any person) in any circle can impact a change in any circle (structurally and policy-wise) and the fact that these changes have to occur through consent from other Circle Members is what makes it different from traditional management.
It may not be revolutionary in that it is only adding a twist in the process of decision making and spreading out power all over the structure, but it sure feels like it’s a major change for most people, including myself. Implementing Holacracy is not easy precisely because it involves habit and mindset changes, and this part, in my opinion, is the revolutionary part. It’s a huge change that not a lot of people want to do just yet.