When not damaged, the defense is undefeatable, but once broken, it becomes a complete failure.
However, the defense of the heavy Star Destroyer still has its basic protective layer, and under normal circumstances, minor damage doesn't matter much.
But this time is not normal; they need to focus on guarding against the Thunderstorm Cannon, so an emergency new defensive shell was added to the Star Destroyers here.
This defensive shell is not strong, but it is enough to deal with the Thunderstorm Cannon. The attack from this cannon draws on the lightning from Thunderstorm Land, making it difficult to cause fatal damage, but it easily causes destruction by percentage.
The shell of the heavy Star Destroyer can be damaged by the Thunderstorm Cannon, and if you replace it with the shell of an Advanced Battleship, it would certainly be the same. Thus, adding a layer of cheap shell to the Star Destroyer is not difficult.
