this is not the only final fantasy game where the bombs are weak against fire. it never seems right that a mob that is based on fire would also be weak to it. however think about it this way. that is the move they do that kills them? self destrust which is also fire based... so it's only logical to also assume that all other fire based attacks would be lethal to them.
These bombs made me eat through two instant warp scrolls because they have an insane aggro range. If you plan on being the "switch hitter" on the east plate, bring some Prism powders and silent oils.