Bruno, I am developer and I worked in traditional companies and now in a holacratic one — and I can assure you: Hierachies of people (traditional companies) and hierachies of work (holacracy) are completly different.
The main difference: Instead of positions we have roles. Positions work like a tree, there is always one node above you and maybe several below you. The person above you may overule any of you decisions and if you got an idea this person may support you or not.
In contrast any member of a holacratic company can empower any number of roles in a number of circles. Tasks that pops up in a circle are assigned to one role, meaning that the person empowering that role makes any decision and cannot (easily) be overruled (though if the performance is bad the role might be assigned to someone else).
So: While your example of a tree works for the circles, it doesn’t work for the roles. They form a dynamic meshed net. While the tree is used for information flow, it is that net that gets things done.