EDIT: typo
This is also why climbers use "dynamic" (elastic) ropes (when climbing, not necessarily for rappelling). Even a very short fall on a static line could be fatal.
1. There hasn't been a crash involving a commercial flight with few survivors in living memory. Either you get something like MH370 where everyone perishes or you get Asiana in SF which had three fatalities.
2. Absolutely baloney about the belt, the worst you can get is a bruise.
And you can get a hell of a lot more than a bruise from a seatbelt due to the magnitude of the forces involved in a plane crash, especially one that [allegedly] killed almost everyone else. Seat belt syndrome is a well-known thing.
The GP's story may or may not be true but your points for it being "extremely unlikely" don't hold water at all.