The difference in a hierarchical org is that the structure is laid out on paper for everyone to see. The power dynamic is explicit rather than implicit.

(At least, it's supposed to be. You do get weird inversions from time to time, when a weak manager is put in charge of a domineering empire-builder, but it's rare - and even in that case, the rest of the org can see the anomaly - as they have a frame of reference to measure it against.)

"Humans are complicated" => that is my uncertainty caveat. We're not absolute-hierarchical nor absolute-flat, in much the same way as we're neither exactly chimp-like nor gorilla-like in terms of monogamy. My observation though (and hardly an original one) is that when we build organisations which try to deny hierarchy, it has a habit of sneaking in thru the back door.