He’s average over the course of a game, but when it counts, he’s bad. On 3rd down, in the 4th quarter, and when it comes to generating big plays. Being bad in those categories is typical of all backups. They need everything else to go perfect for them to win.
Also, he’s not the first backup QB ever to play better than the bottom third of the league. Ryan Fitzpatrick often played middle of the road football, but no team that started him ever went anywhere.