Yeah I see what you're getting at, and a seat belt would probably have saved this man.
To point back to the ASTM standard for a second though, most seatbelts in use on rollercoasters are not allowed to be counted as part of the restraint system for enhancing it's class rating to 5 via redundancy because they can be opened by the rider (6.4.3.9 (4)). The vast majority of intense rides that use them are typically using them as external verification of sufficient restraint position instead of or in addition to the proximity switch.
From an industrial safety perspective the seatbelt feels like more of an administrative control to me than an engineered* control because of the rider ability to bypass. Which an administrative control can prevent accidents, but an engineered control is definitely preferable where feasible.
*For anyone following along, the seatbelts are definitely engineered to handle the loads, just in my opinion industrial safety wouldn't put them in an engineered control category because of aforementioned reasons
