For people who only wear their Invisalign retainers when they are awake and do not clench their teeth, will natural molar extrusion be impeded?

Yes. with invisalign need to wear them at least 22 hours a day. when you take them off specially if there is posterior open contact yes the molars can extrude.
Passive erutipn. Any tooth without an opposing tooth in the other arch will erupt seeking contact. Retainers will prevent this normal physiologic change.