I don't care if he was 120 at the time. That still doesn't make him old enough to have witnessed catapult warfare at the walls of Constantinople!
and had been depending on Austrian and French troops to keep his loyal subjects from running him out of Rome entirely so it is, I suppose, possible that he might have pictured the attack exactly as you described.
You cannot be serious, non-seq. It is an absurdity to suggest that, upon hearing the figurative statement Greek Fire, the pope thought they were referring to catapults flinging pots of sulphuric stuff over the battlements of some medieval fortress! If he was 70 in 1863 that would have made him a boy during the Napoleonic wars that consumed all of Europe, leaving virtually no doubt that he knew of warfare's progression beyond medieval times.
It's a shame that Mr. Mann will never know how lucky he is that we have you to tell him what he was thinking.
Considering that you have already told everyone that he was thinking of catapults flinging pots of boiling sulphur in a medieval seige circa 1863, my offering of meaning to his words is nothing more than a reasonable correction upon the absurdity that you originally assigned to him.