KV asked how Luhmann avoided a hierarchical structure in his Zettelkasten. The answer depends to some extent on how strictly you interpret the word "hierarchy." In some sense, any ordering that has 1 followed by 1.1, 1.1.1, etc. may be interpreted as hierarchical, as "1.1.1" can be viewed as depending on 1. But it does not have to be so interpreted. It could simply indicate a sequence.
However, this is not the kind of hierarchy Luhmann wanted to avoid. What he had in mind is a more substantive hierarchy, where one concept contains other concepts or where all the concepts in a certain domain depend ultimately on one basic concept. Trees represent this kind of hierarchy:
His Zettelkasten does not have this structure of an acyclic graph, but is rather a flatter graph, in which there is not one central node or concept, but in which everything can refer to everything. Like this:
It's more like a network. Luhmann himself used the metaphor of "spider web."
A tree is in fact only one (very specialized) form of a graph, as you can see from this:
Outliners basically follow the tree model and have difficulties accommodating others.
Luhmann's system is not particularly tied to any of these and can accommodate all of them to some extent. Therefore, again, he does not have to avoid any hint of hierarchy.
See also Outlines and Meshes and Outlines and Hypertext. Brainstorm may be viewed as a non-hierarchical outliner.