In small FreeRTOS applications it is unlikely there will be any task pools other...

In small FreeRTOS applications it is unlikely there will be any task pools other than the system task pool.  IotTaskPool_CreateRecyclableSystemJob() is therefore introduced to complement IotTaskPool_CreateRecyclableJob() that does not require the handle of the target task pool to be specified as a parameter.  Likewise IotTaskPool_ScheduleSystemJob() is introduced to complement IotTaskPool_ScheduleJob() for the same reason.
IotTaskPool_CreateSystemTaskPool() calls synchronisation primitives, so cannot be called before the scheduler starts.  Add a configASSERT() to ensure the scheduler is running when it executes.
IotTaskPool_CreateSystemTaskPool() can conceivably be called from multiple different libraries that depend on the thread pool.  In this version _IotSystemTaskPool.running can be used to check the system task pool has not already been created.  If the task pool has been created simply return from IotTaskPool_CreateSystemTaskPool() instead of re-creating it (which would leak memory and leave orphaned tasks).
Call taskENTER_CRITICAL() and taskEXIT_CRITICAL() directly in place of mapping them to TASKPOOL_ENTER_CRITICAL() and TASKPOOL_EXIT_CRITICAL() in the same file.
Rename _timerThread() _timerCallback(), as it is a callback function and not a thread.
Remove the unused flags parameter from _scheduleInternal().