get_timestamp_us is micro second based. If xTaskGetTickCount is nano second based, you just have to multiply 1000, not 1,000,000.
It doesn't have to have very accurate value, but we check it as system time, should be enough to check time like clock.
After changing, and please test it.
I have try to set that timestamp into nano sec. format as the below request:
/* get the timestamp in nanoseconds before calling bsec_sensor_control() */
However, the bvoc result still not change. (Remark: i have attached the setting as below)