If Chappell isn't a top level golfer (and I agree that he isn't), then neither is Justin Thomas, that's for sure. You cannot argue that Thomas is better than Chappell right now, nor can you argue that he's been better over the past 12 months. Chappell's been better in literally every conceivable way. He's the pick.
Also, Chappell lost in the playoff after he made par on the first hole. Moore lasted 4 holes against him. If that's considered "wilting" then your standards are too high. If they fell apart and made bogeys/doubles, then maybe. But they didn't.