People who follow all safety instructions endure fewer light injuries, severe injuries and fatalities. Pilots who joke about seatbelts are just an example of professionals being hilariously wrong (or sardonic) about a topic adjacent to their expertise. Or they were just joking about their usefullness in an uncontrolled crash landing, which is extremely rare.

In this case, a seat belt would have likely saved the person - heavily injuring yourself is quite traumatic (tautologically) which increases the risk of cardiac events. In any case, injuries due to turbulence are extremely common as far as injuries in airliners go.