Then you would know he knocked out countless professional fighters while backing up.
Rope-A-Dope.
The Rope A Dope was a defensive mechanism designed to make punchers like Foreman and Norton punch themselves into fatigue. Once those guys got tired and winded from punching, then Ali would come off the ropes and finish them off. The Rope A Dope was not an offensive mechanism designed to punch anyone out.
Ali only punched out one boxer that I can recall while backing up and that was Sonny Liston. I put the video in this thread.