User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-96.246.232.99-20141123054545/@comment-24468758-20141123061433

It's similar to how some angels were able to help the terrorists without falling. The exorcists just kept quiet to everyone who wouldn't side with them and then they acted with a surprise attack.

PS: Tatsumi wouldn't really stand a chance against Issei as both of their armors increase speed and strength of the wielder but BG is capable to further increase the abilities with boosts whereas Incursio does not have that kind of ability.