AI is most excellent at reading and understanding large codebases and, with some guidance docs, can easily reproduce accurate technical documentation. Divide and conquer.
Reading a large codebase...perhaps if it is not too large. Understanding... why a tool exists, what is the motivation for its design, what the external human systems requirements for successful utilization of the internal facing tools... especially when that knowledge exists only in the memories of a few developers and PMs... not so much.
Deep domain expertise is a long way from AI capability for effective replacement.
Again, nobody would spend a dime to create the technical documentation, even if it could be done somewhat faster with AI support. Also, in my experience AI is not so great explaining the consequences to business processes when documenting code.
Accuracy/faithfulness to the code as written isn't necessarily what you care about though, it's an understanding of the underlying problem. Just translating code doesn't actually help you do that.
