Hello Matthieu!
First of all, thanks for your words about this project! 🙂
Answering your question: Yes, the upper part is only connected with the bottom one using the 3D printed bearing. To be honest, when I designed it I was a little bit worried about the maximum load this 3d printed bearing can hold, but as I did not find a cheap commercial solution I tried and it worked. To be honest, I did not make a proper study about the forces acting on the bearing or a simulation to see the limits of this configuration.
Hope it helps!