This is correct. There is no formalized stack ranking (at least for eng positions) at Google. There's always informal stacking: "does it make sense this person is rated above this other one?" "If we line up all the promotes and don't promotes, does this cut off point make sense)." However, I've seen cases where 75% of the promos made it and cases where it was closer to 30%. It just depends on the group at the time.