Is it solely a matter of duration or not? Why is a pillow deadlier than a simple MMA choke (BJJ, whatever)? How does it work?
The two basic mechanisms when choking are blood and air. A skillful blood choke will knock someone out within seconds, say 2-20. A pillow choke blocks air, which takes much longer ~2? minutes. Death basically occurs when the brain has been deprived of oxygen for minutes. With a pillow, the person's brain is already oxygen starved when they pass out. If you hold a blood choke for minutes after the person goes unconscious, the expectation is this will result in death. This is why you let go. The person is still breathing with a heartbeat and will wake up.
As a martial matter, you always want to cut off blood. This will cause the opponent to go unconscious much faster than if you cut off air, when they will fight back with vigor to restore airflow.