"Candidates express gratitude" is measured by the person who still holds the decision, at the moment the candidate has the most to gain by looking coachable. Someone who genuinely found it useful and someone who is just managing you produce the same reading. An instrument that returns the same value either way isn't measuring the thing.
What's interesting is that the thread already contains the version with the incentive removed: 0plus1 got the feedback attached to a rejection, after the decision, and describes it landing: "if they just asked to clarify some of those points I would've easily explained them." Same feedback, nothing left to lose.
So maybe the closing move is right and the timing is wrong: send it with the outcome, not before it.