That's not how they did it. They had one team read the BIOS source listings in the IBM PC Technical Reference Manual and create a technical specification and a second team take that specification and write a new BIOS [1]. The second team never saw the original code so therefore they could not have copied it.
To do something similar with AI, you really need to train one AI on the source code and then have it explain that code to a second AI that never saw the original code.
They cloned the bios by observing how it behaved and writing code that behaved the same way. Nobody even looked at the bios code.