Although Aiwass has been the "Human Pope" for four months, Arsene still doesn't know what His Holiness looks like.
Even though the news of Aiwass becoming Pope was overshadowed by the arrest and investigation of the former Alice de Hacourt, and didn't get the photos to Irisflower immediately... the scene of Aiwass resisting the Fallen Celestial Marshal was briefly broadcasted to Irisflower. Although only the part of the city near Star Antimony saw it, it was enough for people to take photos and spread them everywhere.
He had only heard people mention that His Holiness the Pope and the Fallen Celestial Marshal looked almost identical—which is actually what people focused on the most. After all, this was too easy to let imaginations run wild.