There is nothing magic about the protection afforded by vaccination. You get vaccinated, your body produces antibodies that circulate among your tissues at some level. Then you are challenged by some number of virus particles. Hopefully, you have produced enough antibodies to sequester the viral load long enough for the other mechanisms to clear them before some number can infect some cells and start an infection. With repeated & sustained exposure, it will be a race between antibody production and the viral load.
In the case of this prison setting, you have 233 individuals plus guards living close quarters continuously. We don't know who was infected first but let's suppose an unvaccinated person originated the initial infection and spread it to some number of unvaccinated people. At some point, the viral load in the unit could be astronomical and sustained virtually continuously. It wouldn't be at all surprising for breakthrough infections to occur in this situation.
We already know that there are practical limits to the effectiveness of monoclonal antibody treatments. Antibody treatment after a certain point is ineffective simply because the infection has become too well established. Why shouldn't be the same for the antibodies produced by vaccination?
The failure of the vaccine system in such an extreme and contrived system doesn't mean that in the case of a vaccinated individual being exposed during a brief, less intense contact the vaccine might very well be perfectly effective.
