The trick with that part of the diagram is that the code is actually dealing with groups of CPUs. So a given group of CPUs might have some that are in dyntick-idle state and others that have somehow avoided passing through a quiescent state, despite the fact that they are online and running. We send a reschedule IPI only to these latter CPUs, not to the dyntick-idle CPUs.
Now that you mention it, this might indeed be a good quick-quiz candidate. :-)