Most likely it was a figurative reference to shells containing explosive chemical mixtures that rain fire and shrapnel down on their targets (i.e. gunpowder).
Like what? What mixture did they have that did that? Canister was short range only, and the Union forces never got close enough to use that. The navy used solid shot and explosive shells in the Civil War. So did the army. These can't be confused with Greek Fire at all so unless the purpose was to be deliberately misleading I can't imagine where the confusion would come from. Considering some of Mr. Mann's other comments that is the most likely explanation.