Are those internal documents in the room with us right now?
No but seriously, most of the software out there is legacy code (don't quote me on that though). IME, legacy code very poorly documented, if anything at all. Sure you could let the LLM extract semantics from the code alone but with old code, arcane hacks and such LLM interpretation can take you only so far. And even then semantics is not always directly translates to business logic.