Thermite was discovered 1893 so it fits the game's timeline. Considering that the armor piercing damage of the hades is on the AOE a shell doesn't fit, I'm thinking some kind of napalm like burning fuse that when it splashes on the target ship mixes with air and ignites the thermite mixed into it. If it has too little time to start burning fully it lacks the temperature to ignite thermite, hence the arming time & moderate direct fire damage.