since i couldn't find any standard resource, here is what i think,
“During context switch” which means the decision to switch the process has already been taken by the scheduler and the dispatcher is already working on context switching, since context switching is a type of interrupt the progress of that particular process must have stopped right after it received an interrupt.
The process is no longer progressing forward, it’s working is halted, the dispatcher is doing everything at this point.
So the processor is working , dispatcher is working everything else is working except for the process which was removed from the active state, hence it is idle.
However the meaning of idle also depends on situation, in general terms idle means not doing anything or not doing anything useful. Like we say the processor is idle, it seems it is not doing anything but it is internally calling HALT and checking for interrupt again and again.
