Fair to say, Duncan was truly sincere at this time—he greatly admired the tenacity and straightforwardness of this Judge. He appreciated her performance in the disaster, and even without this esteem, he deeply valued Fenna as a special "node."
If not for a sufficient coincidence, placing such a "node" among the church's top hierarchy wouldn't have been easy, and to establish even the most basic "acquaintance" would have been even more difficult.
Fenna was incredibly straightforward, and this straightforwardness ensured she would not deny Duncan's credit in sheltering Plunder City-State, deciding that even if personally inconvenienced, she would treat this "great favor" fairly.
However, Duncan's sincerity might come off as rather frightening to others.