Turbulence from storm activity is pretty common in that part of the world and I cannot see any good reason to unbuckle the belt completely unless going to use the toilets.
It may be a hard lesson for some to learn, but I bet all the passengers on this flight will keep them buckled in future.
Cumulonimbus storm clouds can top out above 50,000ft, way higher than normal aircraft cruise levels and are certainly to be avoided.
Way back in 1954 a De Havilland Comet broke up mid flight due to excess stresses on the airframe when flying through a storm.