Air bags do kill without a seat belt, it is a concrete wall when the driver gets to them while still being inflated. Once inflated it can allow to be depressurized on impact by the release cutouts on the back. While the rocket fires to inflate it is very harmful and the srs module cannot tell where you are at the time of inflation so it does not inflate it. Broken noses and burns you hear about are from people who do not have the belts tight while driving. The reason the pretensioners exist is just for that reason which is to correct your seating position not to lessen the impact . the airbag does the rest in tandem.
So like i said , no seat belt, no airbag. At least on any gen 2 system after 97 .