I know you feel that you can freely speak for people long since dead, but it's just as likely that the term was meant to whip up support for the southern cause among those who had no idea of what was really going on.
Do you honestly believe that the pope thought yankee ships were sailing into Charleston harbor, edging up against the city's massive stone walls, and flinging giant pots of boiling sulphuric compounds (almost assuredly with their onboard catapults) over the battlements, with their guardian archers, and onto the civilians from the neighboring serfdoms taking refuge inside? This is 1863 we are talking about, non-seq. Had he wanted to shock the pope with an extremely literal account of the horrors of war he could have easily said "they're firing long range iron shot filled with exploding gunpowder" over our cities, which was absolutely true. The use of the term Greek Fire was a figurative statement that conveyed the exact same thing. NOBODY in their right mind would have responded to that by thinking back to days of catapults, archers, and pots of boiling sulphur. That you, for some unknow and irrational reason do, only further demonstrates the accuracy of my previous statement: You are exhibiting an anal retentive characteristic of Wlatian proportions.