The armour plate is so strong that a shell cannot pierce it either from the outside e.g. fired by an enemy, or from the inside e.g. the shell is kept close to the body, or inside a tank.
Looking at the wider context (I found the sentence), the idea is that the attackers somehow put explosives inside the tank. When they explode, the strength of the armor plate contains the explosion and so causes more damage to the people inside the tank.