Mullin did not speak to the backup/contingency plane per your definition. He was saying that the second airplane that does share duty as AF1 has airframe issues. That means it won’t pass inspection for flight. It started flying in 1987 and was discontinued in 1991. They don’t make replacement parts for it anymore. They have to be custom made. Chances are, the one in the shop will never be used again.
There’s a backup, alternate everywhere they go. It’s like a motorcade in the sky, actually. The vehicles, Secret Service, fighter jets, etc.
bout forgot, also a decoy.