Compare the economy they had in the mid or early war to that in the early thirties or late twenties. Germany was in chaos. The output of their economy was still increasing throughout the allied bombing campaign. Duplication was removed, competition reduced, production lines simplified and available models decreased. Vulnerabilities were addressed and production went up, fast. And yes, by every measure I can think of it wasn't sustainable, but it produced a lot in a short time with extremely difficult conditions.