its because the sides were stainless steel. The explosion was contained by them. Had it been sheet metal it would have blasted out the sides as well. The bed was closed at the time as well and can support an adult jumping on it, but up was still the path of least resistance because of the sides.

You can see the videos the sheriff plays in his press conference, the sides were intact after the explosion.

Edit: So yes, it is very likely that the Cybertruck specifically did limit damage.

Edit: It was also enough force to knock open the closed tail gate.

Edit: You can also see in this picture how it deformed the outer steel on the right. This stuff is 9mm bulltet proof, it takes a lot of energy to deform it. That’s where it would have come out if it was just sheet metal. If it had created an opening it would have let even more out in that direction which would have caused even more damage in that direction, which would have made the opening bigger, letting more out etc.