Earlier quoted context omitted.
This sounds like solid evidence that the Devil is scrupulously fair! He could have just said that his performance was better; his demon backup band would surely have taken his side. I suspect he enjoys the challenge of outdoing mortals, and thus allows them a sporting chance of escape. Note the impressive loophole in this contract! [0] [0] https://news.ycombinator.com/item?id=29127547
An alternative interpretation is that by losing, the devil offers the fiction that a human can succeed without faith in God, damning them.
The Devil clearly can't just take souls without their owner's consent, so he needs to project an air of trustworthiness. There's a serious power asymmetry between mortals and the Devil, and the only higher authority one could think of is God himself. One suspects that God wouldn't easily take the Devil's side, so it behooves (heh) the Devil to be seen as honourable as possible, to ensure smooth and continued soul acquisition.