As stated you're on a tree and somehow within eyesight of the people inside a cathedral. Meaning you're either seeing through the walls, or your inside the church grounds. There's no way to be watching what's happening in the church from a servant's perspective from outside the grounds.
I thought that all our characters were still on the courtyard, since Orazio and Aden were never invited inside. Even then, the tree argument still stands, she never breached the barrier. I'll gladly edit my previous post to remove the part of Assassin seeing Kiyohime, but if I do that, shouldn't the part of the executioner noticing Assassin also be retconed?