yeah that's a true point. But that also means that a human can't code it either. except the person comes from that time so the person has to be over 40 years old and can code or can prompt in the "right" way that the ai can reproduce this.
Humans are perfectly capable of writing code for old systems. Not all of the documentation that exists, exists on the web. If it doesn't, operations can be discovered by reverse-engineering the hardware, mapping the memory, etc. People do this all the time.