NVIC interrupt priorities 5…15 (higher value means lower prio on Cortex-M) are allowed resp. covered by FreeRTOS means you can use FreeRTOS API calls in the corresponding ISRs.
The TIM6 interrupt handler should not use FreeRTOS-API and probably doesn’t.
If you’re using default SysTick for FreeRTOS tick you don’t need to care. FreeRTOS handles this for you (and set the prio to configKERNEL_INTERRUPT_PRIORITY).
See also this posting for a pretty good explanation: