Regardless of how you feel about this realism and danger, this is a prime example of pyrotechnics being misused. Inside a sea container is clearly a violation of training safety "Ground-burst simulators shall NOT be thrown within 15 m of personnel, and 100 m of material, equipment and vehicles." and "Do NOT use the ground-burst simulator in confined spaces such as buildings, vehicles, trenches or weapon pits because these confined areas dangerously increase the effect of the explosion."
Would you feel the same way about the realism if the whistle malfunctioned and you didn't realize that it was in the sea container and you were injured. Or what if a lesser experienced soldier was present and didn't have the common sense or knowledge to seek cover. The permanent damage that can be done to ears, eyes, and the burns that can be received do not make this an acceptable practice. Having a soldier injured by any means in unacceptable, but by mishandling of ammunition makes it even worse and in the long run can cause the rest of the unit more strain when they have to take up the slack of the missing member. This is not realism, this is a violation of drill and training, and it is inexcusable.