I’m not sure how calling vTaskStartScheduler() could effect it because if that function has not been called then there will be no multithreading and calling Delay_Us will result in the processor sitting in Delay_Us until the delay is complete. In other words if the scheduler has not been started then FreeRTOS cannot impact the function.
After the scheduler has been started is a different matter because any task calling Delay_Us can get preempted by any higher priority task, and therefore Delay_Us will not be accurate - the delay time achieved will be either the desired microsecond delay if no context switches occur, or an unknown time if context switches do occur.