i'll have to check the RAW.
If he actually referred to them as 'demons' in front of the Princess, the Pope and everyone in Midland, then, wow, that's a great move on his part.
I mean, there's no way they can get out of this battle without acknowledging the power of Griffith and his Apostles. Just plainly showing them, "Yeah, my army of hell spawn just kicked the shit out of the evil invader (demon) and saved the entire kingdom." puts everyone in the uncomfortable position of plainly seeing what kind of power Griffith has and being unable to do much about it.
It's a brilliant power play. Nobody will ever get to say "Man, something's fishy with Griffith's most powerful soldiers" because everyone will know for sure that that's how he rolls, with butt loads of super powerful demons. He's thrown diplomacy and clever schemes to the wind, banking on his horrifyingly awesome impending victory (and probably a following reign of terror) to do all the talking.
at least, that's where i'm sitting. maybe he'll spin some kind of yarn about how he's so Jesus-like that even demons obey him, but i think he's beyond such deceptions these days.
edit - yeah, that's the kanji for 'demon' alright... some cases it might be 'a spirit' but, well... most often it's used for evil demons, especially when it's War Demon Army. Yeah. Griffith's got balls like Tiger Woods.