Question- wouldn’t the change made by APT5 have been eventually obvious to whoever was originally using the back door? It would stop working for whoever expected the original value to be used, right? How did that go undetected for years?
Optimistic theory: it really was just for emergencies and was not in active use. Pessimistic theory: the NSA had already moved on to bigger and better exploits, but couldn't be bothered to tidy up.