The code base has a lot of documentation in the form of many individual text files. Each describe some isolated aspect of the code in dense, info-rich and not entirely easily consumable (by humans) detail. As numerous as these docs are, the code has many more aspects that lack explicit documentation. And there is a general lack of high-level documentation that tie each isolated doc into some cohesive whole.
I formed a few conclusions about the deepwiki-generated content: First, it is really good where it regurgitates information from the code docs while being rather bad or simply missing for aspects not covered by the provided docs. Second, deepwiki is so-so for providing a high layer of documentation that sort of ties things together. Third, it is highly biased about the importance of various aspects by their code docs coverage.
The lessons I take from this are: deepwiki does better ingesting narrative than code. I can spend less effort on polishing individual documentation (not worrying about how easy it is for humans to absorb). I should instead spend that effort to fill in gaps, both details and to provide higher-level layers of narrative to unify the detailed documentation. I don't need to spend effort on making that unification explicit via sectioning, linking, ordering, etc as one may expect for a "manual" with a table of contents.
In short, I can interpret deepwiki's failings as identifying gaps that need filling by humans while leaning on deepwiki (or similar) to provide polish and some gap putty.