There are 7 deadly sins: pride, greed, wrath, sloth, gluttony, lust and envy.
The pride happens when Johnny tells the devil that he’s the best that’s ever been. The greed is playing for a fiddle of gold. Even Johnny acknowledges that when he says, “It might be a sin but I’ll take your bet…”
So the devil lost the fiddle contest, but that doesn’t mean that he didn’t win Johnny’s soul. It’ll just take a little longer; what’s time for someone immortal like the devil?
The devil is a trickster that will do whatever he can to get you to sin against God. And he did go to Georgia to steal a soul, after all.
The song is written as if it’s told by an onlooker. To suggest that the devil lost is an assumption that the devil is honest and plays by the rules.