It's been a while since I read it myself so it's likely I'll misremember some stuff, but from what I can remember, the central computer that assigned things was due to the extreme scarcity of resources on annares, to the point where they were reliant on following the computer's optimized planning just to avoid starvation. I also seem to recall that the assignments from the computer were still voluntary, it was just that there was that social stigma you mentioned if one opted not to take the assignment given.

In most ways, Annares is structured in a very anarchist way societally, with the exceptions of the council and some positions of power within the education/research sector. The rest of the problems on Annares are mostly social and cultural oppression, which is why I think it's a good example of a work that explores the concept of an Anarchist society that is going awry, making it a good companion piece to the video.