I think you may have gotten that wrong. Airbags are worse if the seatbelt is not worn. The timing for it firing takes into account that a seatbelt restrains the forward movement of the person. No seatbelt means moving forward quicker which means getting the full brunt of the airbag. Best case whiplash, otherwise a broken neck or similar.
At least that is what I have learned and which sounds reasonable.
I dimly seem to remember that there might be differing strategies in place in some parts of the world (maybe something like US vs. Europe?): In some places seatbelts aren't assumed in determining the airbag timings, whereas elsewhere the timing and triggering level are optimised based on the assumption that seatbelts are being used.
At least that is what I have learned and which sounds reasonable.