I believe the airbags will only deploy if the belts are used. In this case the airbag deployed, so the belt was used. It may have been the instant deceleration that killed them especially at that speed. RIP.
Airbags will deploy regardless of seatbelt usage. Newer cars with dual stage airbags will deploy airbags with more force if seatbelts are NOT used.
In high energy accidents even if all the safety systems work as they should, the internal organs cannot be fully supported or restrained and internal injuries from the decelleration forces will occur. In an accident as severe as this, I have known of incidents where the heart is pulled from its supporting tissue by the impact forces so suddenly that the resulting tear in the aorta is instantly fatal in a way that even having emergency medical care present at the scene would not have been able to change the final outcome.
This is only the second Benz I have ever seen where the passenger cell deformed to such a degree that the windshield post and its supporting structure collasped to the degree seen in this accident. The other one was a fatal crash too. Although I am confident that Mercedes builds the safests cars out there I am also aware that the laws of physics cannot be ignored.
In a world where the idiots talking on cell phones while driving outnumber the drunks on the road, I try to give myself the best chance of survival by driving carefully and by driving a Benz. That said, my most recent injury at the hands of another was from being knocked off my feet in a grocery store by a woman pushing a loaded shopping cart while talking on a cell phone.