The question here is whether or not the Sermonic Performance counts as bardic performance for the Banner of the Ancient Kings.
Relevant text:
This ability is similar in all respects to bardic performance as used by a bard of the same level (including interactions with feats, spells, and prestige classes), using Perform (oratory) as the evangelist’s performance skill.
Emphasis mine.
I believe RAI is that the banner should work and recognize the cleric as a bard of the same level.
yes, you're a bard for performances now. Including isn't limited to.